Nephrology

Renal Handling of Urea

July 22, 2017 Laboratory Medicine, Nephrology, Physiology and Pathophysiology, Urology No comments , , , , , , ,

Renal Handling of Urate

Urate, an anion that is the base form of uric acid, provides a fascinating example of the renal handling of organic anions that is particularly important for clinical medicine and is illustrative of renal pathology. An increase in the plasma concentration of urate can cause gout and is thought to be involved in some forms of heart disease and renal disease; therefore, its removal from the blood is important. However, instead of excreting all the urate it can, the kidneys actually reabsorb most of the filtered urate. Urate is freely filterable. Almost all the filtered rate is reabsorbed early in the proximal tubule, primarily via antiporters (URAT1) that exchange urate for another organic anion. Further on the proximal tubule urate undergoes active tubular secretion. Then, in the straight portion, some of the urate is once again reabsorbed. Because the total rate of reabsorption is normally much greater than the rate of secretion, only a small fraction of the filtered load is excreted.

Although urate reabsorption is greater than secretion, the secretory process is controlled to maintain relative constancy of plasma urate. In other words, if plasma urate begins to increase because of increased urate production, the active proximal secretion of urate is stimulated, thereby increasing urate excretion.

Given these mechanisms of renal urate handling, the reader should be able to deduce the 3 ways by which altered renal function can lead to decreased urate excretion and hence increased plasma urate, as in gout: 1) decreased filtration of urate secondary to decreased GFR, 2) excessive reabsorption of urate, and 3) diminished secretion of urate.

Urate, and some other organic solutes, although more membrane permeable in the neutral form, are less soluble in aqueous solution and tend to precipitate. The combination of excess plasma urate and low urinary pH, which converts urate to the neutral uric acid, often leads to the formation of uric acid kidney stones.

Renal Handling of Urea

Urea is a very special substance for the kidney. It is an end product of protein metabolism, waste to be excreted, and also an important component for the regulation of water excretion. Urea differs from all the other organic solutes in several significant ways. 1) There are no membrane transport mechanisms in the proximal tubule; instead, it easily permeates the tight junctions of the proximal tubule where it is reabsorbed paracellularly. 2) Tubular elements beyond the proximal tubule express urea transporters and handle urea in a complex, regulated manner.

Urea is derived from proteins, which form much of the functional and structural substance of body tissues. Proteins are also a source of metabolic fuel. Dietary protein is first digested into its constituent amino acids. These are then used as building blocks for tissue protein, converted to fat or oxidized immediately. During fasting, the body breaks down proteins into amino acids that are used as fuel, in essence consuming itself. The metabolism of amino acids yields a nitrogen moiety (ammonium) and a carbohydrate moiety. The carbohydrate goes on to further metabolic processing, but the ammonium cannot be further oxidized and is a waste product. Ammonium per se is rather toxic to most tissues and the liver immediately converts most ammonium to urea and a smaller, but crucial amount to glutamine. While normal levels of urea are not toxic, the large amounts produced on a daily basis, particularly on a high protein diet, represent a large osmotic load that must be excreted. Whether a person is well fed or fasting, urea production proceeds continuously and constitutes about half of the usual solute content of urine.

The normal level of urea in the blood is quite variable, reflecting variations in both protein intake and renal handling of urea. Over days to weeks, renal urea excretion must match hepatic production; otherwise plasma levels would increase into the pathological range producing a condition called uremia. On a short-term basis (hours to days), urea excretion rate may not exactly match production rate because urea excretion is also regulated for purposes other than keeping a stable plasma level.

The gist of the renal handling of urea is the following: it is freely filtered. About half is reabsorbed passively in the proximal tubule. Then an amount equal to that reabsorbed is secreted back into the loop of Henle. Finally, about half is reabsorbed a second time in the medullary collecting duct. The net result is that about half the filtered load is excreted.

pH Dependence of Passive Reabsorption or Secretion

Many of the organic solutes handled by the kidney are weak acids or bases and exist in both, neutral and ionized forms. The state of ionization affects both the aqueous solubility and membrane permeability of the substance. Neutral solutes are more permeable than ionized solutes. As water is reabsorbed from the tubule, any substance remaining in the tubule becomes progressively more concentrated. And the luminal pH may change substantially during flow through the tubules. Therefore, both the progressive concentration of organic solutes and change in pH strongly influence the degree to which they are reabsorbed by passive diffusion through regions of tubule beyond the proximal tubule.

At low pH weak acids are predominantly neutral, while at high pH they dissociate into an anion and a proton. Imagine the case in which the tubular fluid becomes acidified relative to the plasma, which it does on a typical Western diet. For a weak acid in the tubular fluid, acidification converts much of the acid to the neutral form and therefore, increases its permeability. This favors diffusion out of the lumen (reabsorption). Highly acidic urine tends to increase passive reabsorption of weak acids (and promote less excretion). For many weak bases, the pH dependence is just opposite. At low pH they are protonated cations. As the urine becomes acidified, more is converted to the impermeable charged form and is trapped in the lumen. Less is reabsorbed passively, and more is excreted.

Ammonia and Urea Cycle

July 20, 2017 Gastroenterology, Medicinal Chemistry, Nephrology, Physiology and Pathophysiology No comments , , , , ,

Ammonia (NH3) is a small metabolite that results predominantly from protein and amino acid degradation. It is highly membrane-permeant and readily crosses epithelial barriers in its nonionized form.

Ammonia does not have a physiologic function. However, it is important clinically because it is highly toxic to the nervous system. Because ammonia is being formed constantly from the deamination of amino acids derived from proteins, it is important that mechanisms exist to provide for the timely and efficient disposal fo this molecule. The liver is critical for ammonia catabolism because it is the only tissue in which all elements of the urea cycle, also known as the Krebs-Henseleit cycle, are expressed, providing for the conversion of ammonia to urea. Ammonia is also consumed in the synthesis of nonessential amino acids, and in various facets of intermediary metabolism.

Ammonia Formation and Disposition

Ammonia in the circulation originates in a number of different sites. A diagram showing the major contributors to ammonia levels is shown in 14-1. Note that the liver is efficient in taking up ammonia from the portal blood in health, leaving only approximately 15% to spill over into the systemic circulation.

Intestinal Production

The major contributor to plasma ammonia is the intestine, supplying about 50% of the plasma load. Intestinal ammonia is derived via two major mechanisms. First, ammonia is liberated from urea in the intestinal lumen by enzymes known as ureases. Ureases are not expressed by mammalian cells, but are products of many bacteria, and convert urea to ammonia and carbon dioxide. Indeed, this provides the basis for a common diagnostic test, since H. pylori, which colonizes the gastric lumen and has been identified as a cause of peptic ulcer disease, has a potent urease. Therefore, if patients are given a dose of urea labeled with carbon-13, rapid production of labeled carbon dioxide in the breath is suggestive of infection with this microorganism.

Second, after proteins are digested by either host or bacterial proteases, further breakdown of amino acids generates free ammonia. Ammonia in its unionized form crosses the intestinal epithelium freely, and enters the portal circulation to travel to the liver; however, depending on the pH of the colonic contents, a portion of the ammonia will be protonated to ammonium ion. Because the colonic pH is usually slightly acidic, secondary to the production of short-chain fatty acids, the ammonium is thereby trapped in the lumen and can be eliminated in the stool.

Extraintestinal Production

The second largest contributor to plasma ammonia levels is the kidney. Ammonia is also produced in the liver itself during the deamination of amino acids. Minor additional components of plasma ammonia derive from adenylic acid metabolism in muscle cells, as well as glutamine released from senescent red blood cells.

Urea Cycle

The most important site for ammonia catabolism is the liver, where the elements of the urea cycle are expressed in hepatocytes. Ammonia derived from the sources described earlier is converted in the mitochondria to carbamoyl phosphate, which in turn reacts with ornithine to generate citrulline. Citrulline, in turn, reacts in the cytosol with aspartate, produced by the deamination of glutarate, to yield sequentially arginine succinate then arginine itself. The enzyme arginase then dehydrates arginine to yield urea and ornithine, which returns to the mitochondria and can reenter the cycle to generate additional urea. The net reaction is the combination of two molecules of ammonia with one of carbon dioxide, yielding urea and water.

Urea Disposition

A “mass balance” for the disposition of ammonia and urea is presented in Figure 14-2. As a small molecule, urea can cross cell membranes readily. Likewise, it is filtered at the glomerulus and enters the urine. While urea can be passively reabsorbed across the renal tubule as the urine is concentrated, its permeability is less than that of water such that only approximately half of the filtered load can be reabsorbed. Because of this, the kidney serves as the site where the majority of the urea produced by the liver is excreted. However, some circulating urea may passively back diffuse into the gut, where it is acted on by bacterial ureases to again yield ammonia and (CO2?). Some of the ammonia generated is excreted in the form of ammonium ion; the remainder is again reabsorbed to the handled by the liver once more.

Evaluation of Renal Function

May 3, 2017 Clinical Skills, Critical Care, Nephrology No comments , , , , , , , ,

Assessment of kidney function using both qualitative and quantitative methods is an important part of the evaluation of patients and an essential characterization of individuals who participate in clinical research investigations. Estimating of creatinine clearance has been considered the clinical standard for assessment of kidney function for nearly 50 years, and continues to be used as the primary method of stratifying kidney function in drug pharmacokinetic studies submitted to the United States Food and Drug Administration (FDA). New equations to estimate glomerular filtration rate (GFR) are now used in many clinical settings to identify patients with CKD, and in large epidemiology studies to evaluate risks of mortality and progression to stage 5 CKD, that is , ESKD. Other tests, such as urinalysis, radiographic procedures, and biopsy, are also valuable tools in the assessment of kidney disease, and these qualitative assessments are useful for determining the pathology and etiology of kidney disease.

Quantitative indices of GFR or Clcr are considered the most useful diagnostic tools for identify the presence and monitoring the progression of CKD. These measures can also be used to quantify changes in function that may occur as a result of disease progression, therapeutic intervention, or a toxic insult. It is important to note that the term kidney function includes the combined processes of glomerular filtration, tubular secretion, and reabsorption, as well as endocrine and metabolic functions. This thread critically evaluates the various methods that can be used for the quantitative assessment of kidney function in individuals with normal kidney function, as well as in those with CKD and acute kidney injury (AKI). Where appropriate, discussion regarding the qualitative assessment of the kidney function is also presented, including the role of imaging procedures and invasive tests such as kidney biopsy.

Excretory Function

The kidney is largely responsible for the maintenance of body homeostasis via its role in regulating urinary excretion of water, electrolytes, endogenous substances such as urea, medications, and environmental toxins. It accomplishes this through the combined processes of glomerular filtration, tubular secretion, and reabsorption.

The “intact nephron hypothesis” described by Bricker, more than 40 years ago, proposes that “kidney function” of patients with renal disease is the net result of a reduced number of appropriately functioning nephrons. As the number of nephrons is reduced from the initial complement of 2 million, those that are unaffected compensate; that is, they hyper function. The cornerstone of this hypothesis is that glomerulotubular balance is maintained, such that those nephrons capable of functioning will continue to perform in an appropriate fashion. Extensive studies have indeed shown that single-nephron GFR increases in the unaffected nephrons; thus, the whole-kidney GFR, which represents the sum of the single-nephron GFRs of the remaining functional nephrons, may remain close to normal until there is extensive injury. Based on this, one would presume that a measure of one component of nephron function could be used as an estimate of all renal functions. This, indeed, has been and remains our clinical approach. We estimate GFR and assume secretion and reabsorption remain proportionally intact.

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Filtration

GFR is dependent on numerous factors, one of which is protein load. Bosch suggested that an appropriate comprehensive evaluation of kidney function should include the measurement of “filtration capacity.” Recently, the concept of renal function reserve (RFR) has been defined as the capacity of the kidney to increase GFR in response to physiological or pathologic conditions. This is similar in context to a cardiac stress test. The patient may have no hypoxic symptoms, for example, angina while resting, but it may become quite evident when the patient begins to exercise. Subjects with normal renal function administered an oral or intravenous (IV) protein load prior to measurement of GFR have been noted to increase their GFR by as much as 50%. As renal function declines, the kidneys usually compensate by increasing the single-nephron GFR. The RFR will be reduced in those individuals whose kidneys are already functioning at higher-than-normal levels because of preexisting kidney injury or subclinical loss of kidney mass. Thus, RFR ma be a complementary, insightful index of renal function for many individual with as yet unidentified CKD.

Quantification of renal function (excretory) is not only an important component of a diagnostic evaluation, but it also serves as an important parameter for monitoring therapy directed at the etiology of the diminished function itself, thereby allowing for objective measurement of the success of treatment. Measurement of renal function also serves as a useful indicator of the ability to the kidneys to eliminate drugs from the body. Furthermore, alterations of drug distribution and metabolism have been associated with the degree of renal function. Although several indices have been used for the quantification of GFR in the research setting, estimation of Clcr and GFR are the primary approaches used in the clinical arena.

Secretion

Secretion is an active process that predominantly takes place in the proximal tubule and facilitates the elimination of compounds from the renal circulation into the tubular lumen. Several highly efficient transport pathways exist for a wide range of endogenous and exogenous substances, resulting in renal clearances of these actively secreted entities that often greatly exceed GFR and in some cases approximate renal blood flow. These transporters are typically found among the solute-linked carrier (SLC) and ATB-binding cassette (ABC) super families. Overall, the net process of tubular secretion for drugs is likely a result of multiple secretory pathways acting simultaneously.

Reabsorption

Reabsorption of water and solutes occurs throughout the nephron, whereas the reabsorption of most medications occurs predominantly along the distal tubule and collecting duct. Urine flow rate and physicochemical characteristics of the molecule influence these processes: highly ionized compounds are not reabsorbed unless pH changes within the urine increase the fraction unionized, so that reabsorption may be facilitated.

Endocrine Function

The kidney synthesizes and secretes many hormones involved in maintaining fluid and electrolyte homeostasis. Secretion of renin by the cells of the juxtaglomerular apparatus and production and metabolism of prostaglandins and kinins are among the kidney’s endocrine functions. In addition in response to decreased oxygen tension in the blood, which is sensed by the kidney, erythropoietin is produced and secreted by peritubular fibroblasts. Because these functions are related to renal mass, decreased endocrine activity is associated with the loss of viable kidney cells.

Metabolic Function

The kidney perform a wide variety of metabolic functions, including the activation of vitamin D, gluconeogenesis, and metabolism of endogenous compounds such as insulin, steroids, and xenobiotics. It is common for patients with diabetes and stages 4 to 5 CKD to have reduced requirements for exogenous insulin, and require supplemental therapy with activated vitamin D3 or other vitamin D analogs to avert the bone loss and pain associated with CKD-associated metabolic bone disease. Cytochrome P450, N-acetyltransferase, glutathione transferase, renal peptidases, and other enzymes responsible for the degradation and activation of selected endogenous and exogenous substances have been identified in the kidney. The CYP enzymes in the kidneys are as active as those in the liver, when corrected for organ mass. In vitro and in vivo studies have shown that CYP-mediated metabolism is impaired in the presence of renal failure or uremia. In clinical studies using CYP3A probes in ESRD patients receiving hemodialysis, hepatic CYP3A activity was reported to be reduced by 28% from values observed in age-matched controls; partial correction was noted following the hemodialysis procedure.

Measurement of Kidney Function

The gold standard quantitative index of kidney function is a mGFR. A variety of methods may be used to measure and estimate kidney function in the acute care and ambulatory settings. Measurement of GFR is important for early recognition and monitoring of patients with CKD and as a guide for drug-dose adjustment.

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It is important to recognize conditions that may alter renal function independent of underlying renal pathology. For example, protein intake, such as oral protein loading or an infusion of amino acid solution, may increase GFR. As a result, inter- and intrasubject variability must be considered when it is used as a longitudinal marker of renal function. Dietary protein intake has been demonstrated to correlate with GFR in healthy subjects. The increased GFR following a protein load is the result of renal vasodilation accompanied by an increased renal plasma flow. The exact mechanism of the renal response to protein is unknown, but may be related to extra renal factors such as glucagon, prostaglandins, and angiotensin II, or intra renal mechanisms, such as alterations in tubular transport and tubuloglomerular feedback. Despite the evidence of a “renal reserve,” standardized evaluation techniques have not been developed. Therefore, assessment of a mGFR must consider the dietary protein status of the patient at the time of the study.

Measurement of Glomerular Filtration Rate

  • Measurement of the GFR is most accurate when performed following the exogenous administration of iohexol, iothalamate, or radioisotopes such as technetium-99m diethylenetriamine pentaacetic acid (99mTc-DTPA).

A mGFR remains the single best index of kidney function. As renal mass declines in the presence of age-related loss of nephrons or disease states such as hypertension or diabetes, there is a progressive decline in GFR. The rate of decline in GFR can be used to predict the time to onset of stage 5 CKD, as well as the risk of complications of CKD. Accurate measurement of GFR in clinical practice is a critical variable for individualization of the dosage regimens of renal excreted medications so that one can maximize their therapeutic efficacy and avoid potential toxicity.

The GFR is expressed as the volume of plasma filtered across the glomerulus per unit of time, based on total renal blood flow and capillary hemodynamics. The normal values for GFR are 127 +- 20 mL/min/1.73 m2 and 118 +- 20 mL/min/1.73 m2 in healthy men and women, respectively. These measured values closely approximate what one would predict if the normal renal blood flow were approximately 1.0 L/min/1.73 m2, plasma volume was 60% of blood volume, and filtration fraction across the glomerulus was 20%. In that situation the normal GFR would be expected to be approximately 120 mL/min/1.73 m2.

Optimal clinical measurement of GFR involves determining the renal clearance of a substance that is freely filtered without additional clearance because of tubular secretion or reduction as the result of reabsorption. Additionally, the substance should not be susceptible to metabolism within renal tissues and should not alter renal function. Given these conditions, the mGFR is equivalent to the renal clearance of the solute marker:

GFR = renal Cl = Ae / AUC 0>t

where renal Cl is renal clearance of the marker, Ae is the amount of marker excreted in the urine from time 0 to t, and AUC 0>t is the area under the plasma-concentration-versus-time curve of the marker.

Under steady-state conditions, for example during a continuous infusion of the marker, the expression simplifies to

GFR = renal Cl = Ae / (Css*t)

where Css is the steady-state plasma concentration of the marker achieved during continuous infusion. The continuous infusion method can also be employed without urine collection, where plasma clearance is calculated as Cl = infusion rate / Css. This method is dependent on the attainment of steady-state plasma concentrations and accurate measurement of infusatn concentrations. Plasma clearance can also be determined following a single-dose IV injection with the collection of multiple blood samples to estimate area under the curve (AUC 0>∞). Here, clearance is calculated as Cl = dose/AUC. These plasma clearance methods commonly yield clearance values 10% to 15% higher than GFR measured by urine collection methods.

Several markers have been used for the measurement of GFR and include both exogenous and endogenous compounds. Those administered as exogenous agents, such as inulin, sinistrin, iothalamate, iohexol, and radioisotopes, require specialized administration techniques and detection methods for the quantification of concentrations in serum and urine, but generally provide an accurate measure of GFR. Methods that employ endogenous compounds, such as creatinine or cyst, require less technical expertise, but produce results with greater variability. The GFR marker of choice depends on the purpose and cost of the compound which ranges from $2,000 per vial for radioactive for 125I-iothalamate to $6 per vial for nonradiolabeled iothalamate or iohexol.

Inulin and Sinistrin Clearance

Inulin is a large fructose polysaccharide, obtained from the Jerusalem artichoke, dahlia, and chicory plants. It is not bound to plasma proteins, is freely filtered at the glomerulus, is not secreted or reabsorbed, and is not metabolized by the kidney. The volume of distribution of inulin approximates extracellular volume, or 20% of ideal body weight. Because it is eliminated by glomerular filtration, its elimination half-life is dependent on renal function and is approximately 1.3 hours in subjects with normal renal function. Measurement of plasma and urine inulin concentrations can be performed using high-performance liquid chromatography. Sinistrin, another polyfructosan, has similar characteristics to inulin; it is filtered at the glomerulus and not secreted or reabsorbed to any significant extent. It is a naturally occurring substance derived from the root of the North African vegetable red squill, Urginea maritime, which has a much higher degree of water solubility than inulin. Assay methods for sinistrin have been described using enzymatic procedures, as well as high-performance liquid chromatography with electrochemical detection. Alternatives have been sought for inulin as a marker for GFR because of the problems of availability, high cost, sample preparation and assay variability.

Iothalamate Clearance

Iothalamate is an iodine-containing radio contrast agent that is available in both radiolabeled (125I) and nonradiolabeled forms. This agent is handled in a manner similar to that of inulin; it is freely filtered at the glomerulus and does not undergo substantial tubular secretion or reabsorption. The nonradiolabeled form is most widely used to measure GFR in ambulatory and research settings, and can safely be administered by IV bolus, continuous infusion, or subcutaneous injection. Plasma and urine iothalamate concentrations can be measured using high-performance liquid chromatography. Plasma clearance methods that do not require urine collections have been shown to be highly correlated with renal clearance, making them particularly well-suited for longitudinal evaluations of renal function. These plasma clearance methods require two-compartment modeling approaches because accuracy is dependent on duration of sampling. For example, Agarwal et al. demonstrated that short sampling intervals can overestimate GFR, particularly in patients with severely reduced GFR. In individuals with GFR more than 30 mL/min/1.73 m2 (greater than 0.29 mL/s/m2), a 2-hour sampling strategy yielded GFR values that were 54% higher compared with 10-hour sampling, whereas the 5-hour sampling was 17% higher. In individuals with GFR less than 30 mL/min/1.73 m2, the 5-hour GFR was 36% higher and 2-hour GFR was 126% higher than the 10-hour measurement. The authors proposed a 5- to 7- hour sampling time period with eight plasma samples to be the most appropriate and feasible approach for most GFR evaluations.

Iohexol

Lohexol, a nonionic, low osmolar, iodinated contrast agent, has also been used for the determination of GFR. It is eliminated almost entirely by glomerular filtration, and plasma and renal clearance values are similar to observations with other marker agents: Strong correlations of 0.90 or greater and significant relationships with iothalamate have been reported. These data support iohexol as a suitable alternative marker for the measurement of GFR. A reported advantage of this agent is that a limited number of plasma samples can be used to quantify iohexol plasma clearance. For patients with a reduced GFR more time must allotted – more than 24 hours if the eGFR is less  than 20 mL/min.

Radiolabeled Markers

The GFR has also been quantified using radiolabeled markers, such as 125I-iothalamate, 99mTc-DPTA, and 51Cr-ethylenediaminetetraacetic acid. These relatively small molecules are minimally bound to plasma proteins and do not undergo tubular secretion or reabsorption to any significant degree. 125I-iothalamate and 99mTc-DPTA are used in the United States, whereas 51Cr-EDTA is used extensively in Europe. The use of radiolabeled markers allows one to determine the individual contribution of each kidney to total renal function. Various protocols exist for the administration of these markers and subsequent measurement of GFR using either plasma or renal clearance calculation methods. The non renal clearance of these agents appears to be low, suggesting that plasma clearance is an acceptable technique except in patients with severe renal insufficiency (GFR less than 30 mL/min). Indeed, highly significant correlations between renal clearance among radiolabeled markers has been demonstrated. Although total radioactive exposure to patients is usually minimal, use of these agents does require compliance with radiation safety committees and appropriate biohazard waste disposal.

Optical Real-Time Glomerular Filtration Rate Markers

A clinically applicable technique to rapidly measure GFR, particularly in critically ill patients with unstable kidney function, is highly desirable. The currently available GFR measurement approaches, as outlined above, are technically demanding, time-consuming, and often cost-prohibitive. Research is underway to develop rapid, accurate, safe, and inexpensive techniques to address this need.

Creatinine

Although the measured (24-hour) CLcr has been used as an approximation of GFR for decades, it has limited clinical utility for a multiplicity of reasons. Short-duration witnessed mCLcr correlates well with mGFR based on iothalamate clearance performed using the single-injection technique. In a multicenter study of 136 patients with type 1 diabetic nephropathy, the correlations of simultaneous mCLcr, and 24-hour CLcr (compared to CLiothalamate) were 0.81 and 0.49, respectively, indicating increased variability with the 24-hour clearance determination. In a selected group of 110 patients, measurement of a 4-hour CLcr during water diuresis provided the best estimate of the GFR as determined by the CLiothalamate. Furthermore, the ratio of CLcr to CLiothalamate did not appear to increase as the GFR decreased. These data suggest that a short collection period with a water diuresis may be the best CLcr method for estimation of GFR.

A limitation of using creatinine as a filtration marker is that it undergoes tubular secretion. Tubular secretion arguments the filtered creatinine by approximately 10% in subjects with normal kidney function. If the nonspecific Jaffe reaction is used, which overestimates the Scr by approximately 10% because of the noncreatinine chromogens, then the measurement of CLcr is a very good measure of GFR in patients with normal kidney function. Tubular secretion, however, increases to as much as 100% in patients with kidney disease, resulting in mCLcr values that markedly overestimate GFR. For example, Bauer et al. reported that the CLcr-to-CLinulin ratio in subjects with mild impairment was 1.20; for those with moderate impairment, it was 1.87; and in those with severe impairment, it was 2.32. Thus, a mCLcr is a poor indicator of GFR in patients with moderate to severe renal insufficiency, that is, stages 3 to 5 CKD.

Because cimetidine blocks the tubular secretion of creatinine the potential role of several oral cimetidine regimens to improve the accuracy and precision of mCLcr as an indicator of GFR has been evaluated. The CLcr-to-CLDPTA ratio declined from 1.33 with placebo to 1.07 when 400 mg of cimetidine was administered four times a day for 2 days prior to and during the clearance determination. Similar results were observed when a single 800-mg dose of cimetidine was given 1 hour prior to the simultaneous determination of CLcr and CLiothalamate; the ratio of CLcr to CLiothalamate was reduced from a mean of 1.53 to 1.12. Thus a single oral dose of 800 mg of cimetidine should provide adequate blockade of creatinine secretion to improve the accuracy of a CLcr measurement as an estimate GFR in patients with stage 3 to 5 CKD.

To minimize the impact of diurnal variations in Scr on CLcr, the test is usually performed over a 24-hour period with the plasma creatinine obtained in the morning, as long as the patient has stable kidney function. Collection of urine remains a limiting factor in the 24-hour CLcr because of incomplete collections, and interconversion between creatinine and creatine that can occur if the urine is not maintained at a pH less than 6.

Estimating of Glomerular Filtration Rate

Because of the invasive nature and technical difficulties of directly measuring GFR in clinical settings, many equations for estimating GFR have been proposed over the past 10 years. A series of related GFR estimating equations have been developed for the primary purpose of identifying and classifying CKD in many patient populations. The initial equation was derived from multiple regression analysis of data obtained from the 1,628 patients enrolled in the Modification of Diet in Renal Disease Study (MDRD) where GFR was measured using the renal clearance of 125I-iothalamate methodology. A four-variable version of the original MDRD equation (MDRD4), based on plasma creatinine, age, sex, and race, was shown to provide a similar estimate of GFR results when compared to a six-variable equation predecessor. However, this equation was shown to be inaccurate at GFR more than 60 mL/min/1.73 m2, for reasons not associated with standardization of Screening assay results. A recent study conducted by the FDA compared the eGFR estimated by the MDRD4 equation to the CLcr estimated by the Cockcroft-Gault equation in 973 subjects enrolled in pharmacokinetic studies conducted for new chemical entities submitted to the FDA from 1998 to 2010. The MDRD4 eGFR results consistently overestimated the CLcr calculated by the CG method. The FDA investigators concluded that “For patients with advanced age, low weight, and modestly elevated serum creatinine concentration values, further work is needed before the MDRD equations can replace the CG equation for dose adjustment in approved product information labeling.”

A single eGFR equation may not be best suited for all populations, and choice of equation has been shown to impact CKD prevalence estimates. This has led to a revitalized interest in the development of new equations to estimate GFR. The newest equations to be proposed for the estimation of GFR have been derived from wider CKD populations than the MDRD study, and include the CKD-EPI and the Berlin Initiative Study (BIS). The CKD-EPI equation was developed from pooled study data involving 5,500 patients, with mean GFR values of 68 +- 40 mL/min/1.73 m2. It has been reported that the CKD-EPI equation is less biased but similarly imprecise compared to MDRD4.

CKD-EPI Equation

The CKD-EPI study equation was compared to the MDRD equation using pooled data from patients enrolled in research or clinical outcomes studies, where GFR was measured by any exogenous tracer. The results of the study indicated that the bias of CKD-EPI equation was 61% to 75% lower than the MDRD equation for patients with eGFR of 60 to 119 mL/min/1.73 m2. Based on these findings, the CKD-EPI equation is most appropriate for estimating GFR in individuals with eGFR values more than 60 mL/min/1.73 m2. Both KDOQI and the Australasian Creatinine Consensus Working Groups now recommend that clinical laboratories switch from the MDRD4 to CKD-EPI for routine automated reporting. If one’s clinical lab does not automatically calculate eGFR using the CKD-EPI, it becomes a bit of a challenge since the equation requires a more complex algorithm than the MDRD equation.

Limitations of the pooled analysis approach used to develop the MDRD and CKD-EPI equations include the use of different GFR markers between studies, different methods of administration of the GFR markers and different clearance calculations. These limitations may partly explain the reduced accuracy observed with the MDRD4 equation at GFR values more than 60 mL/min/1.73 m2. Additionally, a recent inspection of the MDRD GFR study data showed that large intrasubject variability in GFR measures was a likely contributor to the inaccuracy of the gold standard method that was used to create the MDRD equation.

Cystatin C-Based Equations

Addition of serum cysC as a covariate in equations to estimate GFR has been employed as a means to improve creatinine-based estimations of GFR that historically were limited to the following variables: lean body mass, age, sex, race, and Scr.

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  • Alb, serum albumin concentration (g/dL); BUN, blood/serum urea nitrogen concentration (mg/dL);CKD, chronic kidney disease; cysC, cystatin C; eGFR, estimated glomerular filtration rate; Scr, serum or plasma creatinine (mg/dL).
  • k is 0.7 for females and 0.9 for males, alpha is -0.329 for females and -0.411 for males, min indicates the minimum of Screening/k or 1, and max indicates the maximum of Scr/k or 1.

A significant limitation of serum cysC as a renal biomarker is the influence of body mass on serum concentrations. When using a serum cyst-based estimate of GFR, which incorporates the serum cysC, age, race, and sex, a higher prevalence of CKD was reported in obese patients when compared to the MDRD4 equation. In a recent retrospective analysis of over 1,000 elderly individuals (mean age 85 years) enrolled in Cardiovascular Health Study, GFR was estimated using the CKD-EPI and CKD-EPI-cysC equation, specifically equation 9 in Table e42-6. In this population, all-cause mortality rates were significantly different between equations, suggesting that cysC does not accurately predict mortality risk in patients with low Screening, reduced muscle mass, and malnutrition. The combined use of serum cysC and creatinine in modified CKD-EPI equations has recently been reported. The CKD-EPIcreatinine_cystatin C, equation 10 in Table e42-6 is now recommended for use in patients where unreliable serum creatinine values are anticipated, such as extremes in body mass, diet, or creatinine assay interferences.

Liver Disease

Evaluation of renal hemodynamics is particularly complicated in patients with liver disease and cirrhosis, where filtration fraction is associated with the degree of ascites, renal artery vasoconstriction, and vascular resistance. The estimation of CLcr or GFR can be problematic in patients with preexisting liver disease and renal impairment. Lower-than-expected Scr values may result from reduced muscle mass, protein-poor diet, diminished hepatic synthesis of creatine (a precursor of creatinine), and fluid overload can lead to significant overestimation of CLcr.

Evaluations of new eGFR equations for use in patients with liver disease have yield mixed results. In summary, renal function assessment in patients with hepatic disease should be performed by measuring glomerular filtration, and GFR estimation equations that combine creatinine and cysC are preferred.

Creatinine Clearance Estimation – Non-Steady State

March 8, 2017 Clinical Skills, Laboratory Medicine, Nephrology, Pharmacokinetics, Practice 2 comments , ,

Using non-steady-state serum creatinine values to estimate creatinine clearance is difficult, and a number of approaches have been proposed. The author use Equation 1 below to estimate creatinine clearance when steady-state conditions have not been achieved.

ClCr (mL/min) = { (Production of Creatinine in mg/day) – [(SCr 2 – SCr 1)(V Cr) / t]*(10 dL/L)} * [(1000 mL/L) / (1440 min/day)] / [(SCr 2)(10 dL/L)] [Equation 1]

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The daily production of creatinine in milligram is calculated by multiplying the daily production value in mg/kg/day from Table 5 by the patient’s weight in kg. The serum creatinine values in Equation 1 are expressed in units of mg/dL; t is the number (or fraction) of days between the first serum creatinine measurement (SCr1) and the second (SCr2). The volume of distribution of creatinine (Vcr) is calculated by multiplying the patient’s weight in kg times 0.65 L/kg. Equation 1 (or 79) is essentially a modification of the mass balance equation (we will discuss it in another thread later).

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where the daily production of creatinine in milligram has replaced the infusion rate of the drug and the second serum creatinine value replaced C ave. The second serum creatinine is used primarily because Equation 1 is most commonly applied when creatinine clearance is decreasing (serum creatinine rising), and using the higher of the two serum creatinine values results in a lower, more conservative estimate of renal function. Some have suggested that the iterative search process, as represented by the combination of Equation 28 and 37 (won’t be discussed here; if needed, please contact Tom for detail), be used:

Screen Shot 2017 03 08 at 9 29 36 PMwhere C2 represents SCr2, and C represents SCr1. (S)(F)(Dose/tau) represents the daily production of creatinine, and t represents the time interval between the first and second serum creatinine concentrations. Cl represents the creatinine clearance with the corresponding elimination rate constant K being Cl/V or the creatinine clearance divided by the creatinine volume of distribution. As discussed previously, the solution would require an iterative search, and the inherent errors in the calculation process probably do not warrant this type of calculation.

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Although Equation 1 (or Equation 79) can be used to estimate a patient’s creatinine clearance when a patient’s serum creatinine is rising or falling, there are potential problems associated with this and all other approaches using non-steady-state serum creatinine values. First, a rising  serum creatinine concentration may represent a continually declining renal function. To help compensate for the latter possibility, the second creatinine (SCr2) rather than the average is used in the denominator of Equation 1/79. Furthermore, there are non-renal routes of creatinine elimination that become significant in patients with significantly diminished renal function. Because as much as 30% of a patient’s daily creatinine excretion is the result of dietary intake, the ability to predict a patient’s daily creatinine production in the clinical setting is limited. One should also consider the potential errors in estimating creatinine production for the critical ill patient, the errors in serum creatinine measurements, and the uncertainty in the volume of distribution estimate for creatinine. Estimating creatinine clearance in a patient with a rising or falling serum creatinine should be viewed as a best guess under difficult conditions, and ongoing reassessment of the patient’s renal function is warranted.

Creatinine Clearance Estimation – Steady State

March 4, 2017 Clinical Skills, Nephrology, Pharmacokinetics No comments , , , , , , , , , ,

PharmacokineticsGraphDifferent from most drug administration regimens, creatinine is constantly produced and released into plasma by the body muscle mass. Because many drugs are partially or totally eliminated by the kidney, an accurate estimation of renal function is an important component in the application of pharmacokinetics to designing drug therapy regimens. Creatinine clearance as determined by a urine collection and corresponding plasma sample is considered by many clinicians to be the most accurate test of renal function. In the clinical setting, the time delay and the difficulty in obtaining the 24-hour creatinine collection limit the utility of the 24-hour urine collection. In addition, all too often, the urine collection is inaccurate because a portion is accidentally discarded or the time of collection is shorter or longer than requested. Perhaps, the most common error is an incomplete collection, which will result in an underestimation of renal function. Because decisions with regard to drug dosing must often be made quickly, several authors have suggested a variety of methods by which creatinine clearance (ClCr) can be estimated using a serum creatinine value. The most accurate of these equations include serum creatinine, body weight or size, age, and gender.

Creatinine Pharmacokinetics

The pharmacokinetics of creatinine is presented in far more detail elsewhere, but a brief overview is necessary. Creatinine is a metabolic by-product of muscle, and its rate of formation (RA) is primarily determined by an individual’s muscle mass or lean body weight. It varies, therefore, with age (lower in the elderly) and gender (lower in the females). For any given individual, the rate of creatinine production is assumed to be constant. Once creatinine is released from muscle into plasma, it is eliminated almost exclusively by renal glomerular filtration. Any decrease in the glomerular filtration rate ultimately results in a rise in the serum creatinine level until a new steady state is reached and the amount of creatinine cleared per day equals the rate of production. In other words, at steady state, the rate in must equal the rate out. Since the rate of creatinine production remains constant even when renal clearance diminishes, the serum creatinine must rise until the product of the clearance and the serum creatinine again equals the rate of production.

Creatinine, RA = RE

Estimating Creatinine Clearance from Steady-State Serum Creatinine Concentrations

Basic Rationale

The degree to which a steady-state serum creatinine rises is inversely proportional to the decrease in creatinine clearance. Therefore, the new creatinine clearance can be estimated by multiplying a normal ClCr value by the fractional change in the serum creatinine: normal SCr/patient’s SCrss. For the 70-kg man, it can be assumed that the normal SCr is 1.0 mg/dL and that the corresponding ClCr is 120 mL/min.

New ClCr = (120 mL/min) [1 mg/dL / SCr ss] (Equation 1)

On the basis of this concept, one can see that each time the serum creatinine doubles, the creatinine clearance falls by half and that small changes in the serum at low concentrations are of much greater consequence than equal changes in the serum creatinine at high concentrations. To illustrate, if a patient with a normal serum creatinine of 1.0 mg/dL is reported to have a new steady-state serum creatinine of 2 mg/dL, the creatinine clearance has decreased from 120 to 60 mL/min. However, if a patient with chronic renal dysfunction has a usual serum creatinine of 4 mg/dL (ClCr = 30 mL/min), a similar 1.0 mg/dL increase in the serum creatinine to 5 mg/dL would result in a small drop in the ClCr (6 mL/min) and a new clearance value of 24 mL/min. However, at some point even small changes in ClCr can be physiologically significant to the patient. As an example, for a patient with a creatinine clearance of 100 mL/min to have their renal function decline by 10 mL/min is of very little consequence, but for a patient with a creatinine clearance of 15 mL/min, a 10 mL/min decrease would probably change their clinical status from a patient with very poor renal function to a patient who would require dialysis.

The estimation of ClCr from SCr ss alone is reasonably satisfactory as long as the patient’s daily creatinine production is average (i.e., 20 mg/kg/day); the patient weighs approximately 70 kg and the serum creatinine is at steady state (i.e., not rising or falling). These conditions are usually present in the young healthy adult, but young healthy adults are not the typical patients for whom pharmacokinetic manipulations are most useful.

Adjusting to Body Size: Weight or Body Surface Area

To account for any changes in creatinine production and clearance that may result from a difference in body size, Equation 1 can be modified to compensate for any deviation in BSA from the 70-kg patient (1.73 m2):

The patient’s BSA can be obtained from a nomogram, estimated from Equation 2:

BSA in m2 = [(Patient’s Weight in Kg / 70 kg)^0.7]*(1.73 m2)

or calculated from the following equation:

BSA in m2 = (W^0.425)(H^0.725)*0.007184

where BSA is in meters squared (m2), W is weight in kilograms, and H is the patient’s height in centimeters.

A disadvantage of using only weight or BSA is that the elderly or emaciated patients who have a reduced muscle mass do not have a “normal” creatinine clearance of 120 mL/min/1.73 m2 with a serum creatinine value of 1.0 mg/dL. For this reason, it may be erroneous to assume that a SCr of 1.0 mg/dL is indicative of a creatinine clearance of 120 mL/min/1.73 m2 in these individuals.Screen Shot 2017 03 02 at 10 47 36 PM

On average, as patients age, their muscle mass represents a smaller proportion of their total weight and creatinine production is decreased (Table 5). There are a number of equations that consider age, gender, body size, and serum creatinine when calculating or estimating creatinine clearance for adults. Although all these methods are similar and equivalent in clinical practice, the most common method used by clinicians is probably the one proposed by Cockcroft and Gault.

ClCr for males (mL/min) = (140 – Age)(Weight) / [(72)(SCr ss)] [Equation 3]

ClCr for females (mL/min) = (0.85)(140 – Age)(Weight) / [(72)(SCr ss)] [Equation 4]

where age is in years, weight is in kg, and serum creatinine is in mg/dL. Equation 3 and 4 calculate creatinine clearance as mL/min for the patient’s characteristics entered into the equation.

The two most critical factors to consider when using Equation 3 and 4 are the assumptions that the serum creatinine is at steady state and the weight, age, and gender of the individual reflect normal muscle mass. For example, when estimating a creatinine clearance for an obese patient, an estimate of the non-obese or ideal body weight (IBW) should be used in Equation 3 and 4. This estimate can be based on IBW tables or the following equations.

TBW Significantly Larger than IBW

Ideal Body Weight for males in kg = 50 + (2.3)(Height in Inches > 60) [Equation 5]

Ideal Body Weight for female in kg = 45 + (2.3)(Height in Inches > 60) [Equation 6]

It should be pointed out, however, that an IBW derived from a patient’s height, as in Equation 5 and 6, may not represent the actual non-obese weight of a patient. Although there are some potential flaws in estimating the non-obese weight from height, the IBW is usually preferable to using the actual weight [total body weight (TBW)] when a patient is markedly obese. As a clinical guideline, one approach is to make an adjustment for IBW if the patient’s actual weight is > 120% of their IBW.

There are studies indicating that TBW overestimates and IBW underestimates renal function in the morbidly obese patient. It has been suggested that an adjusted body weight between IBW and TBW be used to estimate renal function in obese individuals. While this adjustment factor is variable, 40% of the excess weight is commonly used:

Adjusted Body Weight = IBW + (0.4)(TBW – IBW) [Equation 7]

where IBW is the patient’s ideal body weight in kg as calculated from Equation 5 and 6, and TBW is the patient’s total body weight in kg.

There are other factors not considered in these equations for IBW and Adjusted Body Weight. As an example, in patients with extensive spacing of fluid (i.e., edema or ascites), the liters (kilograms) of excess third-space fluid should probably not be included in the patient’s estimate of TBW. As an example, consider a 5-foot 4-inch male patient weighting 75 kg and having an estimated 15 kg of edema and ascitic fluid. Using the patient’s height (64 inches) and weight (75 kg) might suggest that the patient is more than 120% over his IBW and therefore “clinically obese” for the purposes of doing pharmacokinetic calculations.

For this patient, IBW = 59.2, TBW/IBW = 127%. However, the patient is not obese but rather has a significant amount of interstitial fluid accumulated. This is obvious if we subtract the excessive third-space fluid weight of 15 kg from his total weight of 75 kg, resulting in a weight of 60 kg. Clearly, the difference between the “non-excess third-space fluid weight” of 60 kg and the estimated IBW of 59.2 kg is so small that the patient would not be considered clinically obese.

Likewise, when calculating an Adjusted Body Weight, it would be the patient’s weight minus any significant third-space fluid weight that would be used in Equation 7. The excessive third-space fluid weight may or may not be important to consider in making pharmacokinetic calculations. As an example, significant third-space fluid does contribute to the apparent volume of distribution for some drugs, but is unlikely to be an important contributor to volume of distribution if the apparent volume of distribution is large or if there is significant plasma protein binding.

Third-space fluid weight is unlikely to contribute to and should not be used when initial estimates of clearance are made. However, while not directly influencing clearance, it is possible that the presence of ascites or edema may indicate the presence of a disease process that is known to alter clearance.

TBW Significantly Smaller than IBW

Patients who weigh significantly less than their IBW or are emaciated also require special consideration when estimating renal function. While it may seem counterintuitive, a creatinine clearance calculated for an emaciated subject using the patient’s weight also tends to over predict the patient’s creatinine clearance. This is because patients who are emaciated tend to have a disproportionally greater loss in muscle mass than TBW. Consequently, serum creatinine in the denominator of Equation 3 and 4 decreases more than the weight in the numerator, resulting an overestimate of creatinine clearance. For this reason, if the patient’s actual weight is less than their IBW, the actual weight should be used when calculating creatinine clearance in emaciated subjects. Even then, the creatinine clearance is likely to be overestimated.

Low Serum Creatinine Level

In addition, it has been suggested that when serum creatinine values are < 1.0 mg/dL, more accurate predictions of creatinine clearance can be obtained if these levels are upwardly adjusted or normalized to a value of 1.0 mg/dL. This suggestion is based on the assumption that low serum creatinine values are related to small muscle mass and a decreased creatinine production rather than to an unusually large creatinine clearance. It is a common practice for clinicians to normalize serum creatinine values < 1 to 1 mg/dL. However, there is evidence suggesting that using the actual serum creatinine values of < 1 mg/dL result in more accurate estimates of creatinine clearance. Because of this continuing controversy and the difficulty in estimating creatine clearance accurately, it is important to use clinical judgement in evaluation the risk versus the benefit of drug therapy. When a serum creatinine of < 1mg/dL is used in Equation 3 and 4, most clinicians would recommend setting an upper limit for creatinine clearance. As an example, a 50-year-old man weighing 60 kg with a serum creatinine of 0.5 mg/dL would have a calculated creatinine clearance of 150 mL/min if the serum creatinine of 0.5 mg/dL is used. And a value of 75 mL/min if the serum creatinine is normalized to 1 mg/dL.

Even if the first method is used, many clinicians would suggest that an upper limit for a calculated creatinine clearance should be set at somewhere near 120 mL/min. Of course in specific situations (e.g., very large, non-obese, young healthy male patient), a creatinine clearance of more than 120 mL/min might be appropriate to consider. Therefore, whether to normalize a patient’s serum creatinine and whether there should be some upper limit for the calculated value of creatinine clearance should be dictated by clinical judgement rather than a specific rule.

Estimating Time to Reach a Steady-State Serum Creatinine Level

All the above methods for estimating ClCr require a steady-state serum creatinine concentration. When a patient’s renal function suddenly changes, some period of time will be required to achieve a new steady-state serum creatinine concentration. In this situation, it is important to be able to estimate how long it will take for the SCr to reach steady state. If a rising serum creatinine is used in any of the previous equations, the patient’s creatinine clearance will be overestimated.

As presented earlier, half-life is a function of both the volume of distribution and clearance. If the volume of distribution of creatinine (0.5 to 0.7 L/kg) is assumed to remain constant, the time required to reach 90% of steady state in patient with normal renal function is less than 1 day. As an example, the average 70-kg patient with a creatinine clearance of 120 mL/min (7.2 L/hr) with a volume of distribution for creatinine of 45.5 L (0.65 L/kg) would be expected to have a creatinine t1/2 of 4.4 hours.

Under these conditions, 90% of steady state should be achieved in approximately 15 hours (3.3 t1/2s). However, if the same patient had a creatinine clearance of 10 mL/min (0.6 L/min), the creatinine t1/2 would be 52.5 hours and more than a week would be required to ensure that steady state had been achieved. One useful approach, that helps clinicians to make relatively rapid assessments of SCr, is to remember that as a drug (in this case creatinine) concentration is accumulating toward steady state, half of the total change will occur in the first half-life. Therefore, two serum creatinine concentrations obtained several hours apart (8 to 12 hours) that appear to be similar (i.e., not increasing or declining significantly) and that represent reasonably normal renal function probably represent steady-state conditions. As renal function declines, proportionately longer intervals between creatinine measurements are required to assure that steady-state conditions exits.

In clinical practice, patients occasionally have a slowly increasing serum creatinine. As an example, a patient might have the following serum creatinine concentrations on 4 consecutive days: 1, 1.2, 1.6, and 1.8 mg/dL. First, it should be recognized that the increase in serum creatinine from day 1 to day 2 could be due to assay error alone, as the absolute error for most creatinine assays is +- 0.1 to 0.2 mg/dL. Also, given that the t1/2 of creatinine at concentrations in the range of 1 to 2 mg/dL is approximately 4 to 8 hours, steady state should have been achieved in the first day. Therefore, the continued increase in serum creatinine probably reflects ongoing changes in creatinine clearance over the 4 days. The difficult clinical issue is not what the creatinine clearance is on each of the 4 days, but rather what it will be tomorrow, what is the cause, and how to prevent or minimize the ongoing renal damage.