Adherence

Some Critical Notices Should Knowing When Using Warfarin

June 30, 2017 Anticoagulant Therapy, Hematology, Laboratory Medicine No comments , , , , , , , , , , , ,

PT/INR and Anticoagulation Status

For the vast majority of patients        , monitoring is done using the prothrombin time with international normalized ratio (PT/INR), which reflects the degree of anticoagulation due to depletion of vitamin K-dependent coagulation. However, attention must be paid that the PT/INR in a patient on warfarin may note reflect the total anticoagulation status of the patient in certain settings:

  • First few day of warfarin initiation

The initial prolongation of the PT/INR during the first one to three days of warfarin initiation does not reflect full anticoagulation, because only the factor with the shortest half-life is initially depleted; other functional vitamin K-dependent factors with longer half-lives (e.g., prothrombin) continues to circulate. The full anticoagulation effect of a VKA generally occurs within approximately one week after the initiation of therapy and results in equilibrium levels of functional factors II, IX, and X at approximately 10 to 35 percent of normal.

  • Liver disease

Individuals with liver disease frequently have abnormalities in routine laboratory tests of coagulation, including prolongation of the PT, INR, and aPTT, along with mild thrombocytopenia, elevated D-dimer, especially when liver synthetic function is more significantly impaired and portal pressures are increased. However, these tests are very poor at predicting the risk of bleeding in individuals with liver disease because they only reflect changes in procoagulant factors.

  • Baseline prolonged PT/INR

Some patients with the antiphospholipid antibody syndrome (APS) have marked fluctuations in the INR that make monitoring of the degree of anticoagulation difficult.

Time in the Therapeutic Range (TTR)

For patients who are stably anticoagulated with a VKA, the percentage of time in the therapeutic range (TTR) is often used as a measure of the quality of anticoagulation control. TTR can be calculated using a variety of methods. The TTR reported depends on the method of calculation as well as the INR range considered “therapeutic.” A TTR of 65 to 70 percent is considered to be a reasonable and achievable degree of INR control in most settings.

Factors Affecting the Dose-Response Relationship Between Warfarin and INR

  • Nutritional status, including vitamin K intake
  • Medication Adherence
  • Genetic variation
  • Drug interactions
  • Smoking and alcohol use
  • Renal, hepatic, and cardiac function
  • Hypermetabolic states

In addition, female sex, increased age, and previous INR instability or hemorrhage have been associated with a greater sensitivity to warfarin and/or an increased risk of bleeding.

Dietary Factors

Vitamin K intake – Individuals anti coagulated with warfarin generally are sensitive to fluctuations in vitamin K intake, and adequate INR control requires close attention to the amount of vitamin K ingested from dietary and other sources. The goal of monitoring vitamin K intake is to maintain a moderate, constant level of intake rather than to eliminate vitamin K from the diet. Specific guidance from anticoagulation clinics may vary, but a general principle is that maintaining a consistent level of vitamin K intake should not interfere with a nutritious diet. Patients taking warfarin may wish to be familiar with possible sources of vitamin K (in order to avoid inconsistency).

Of note, intestinal microflora produce vitamin K2, and one of the ways antibiotics contribute to variability in the prothrombin time/INR is by reducing intestinal vitamin K synthesis.

Cranberry juice and grapefruit juice have very low vitamin K content but have been reported to affect VKA anticoagulation in some studies, and some anticoagulation clinics advise patients to limit their intake to one or two servings (or less) per day.

Medication Adherence

Medication adherence for vitamin K antagonists can be challenging due to the need for frequent monitoring and dose adjustments, dietary restrictions, medication interactions, and, in some cases, use of different medication doses on different days to achieve the optimal weekly intake. Reducing the number of medications prescribed may be helpful, if this can be done safely.

Drug Interactions

A large number of drugs interact with vitamin K antagonists by a variety of mechanisms, and additional interacting drugs continue to be introduced. Determine clinically important drug interactions is challenging because the evidence substantiating claims for some drug is very limited; in other cases, the evidence is strong but the magnitude of effect is small. Patients should be advised to discuss any new medication or over-the-counter supplement with the clinician managing their anticoagulation, and clinicians are advised to confirm whether a clinically important drug-drug interaction has been reported when introducing a new medication in a patient anticoagulated with a VKA.

Smoking and Excess Alcohol

The effect of chronic cigarette smoking on warfarin metabolism was evaluated in a systematic review and that included 13 studies involving over 3000 patients. A meta-analysis of the studies that evaluated warfarin dose requirement found that smoking increased the dose requirement by 12 percent, corresponding to a requirement of 2.26 additional mg of warfarin per week. However, two studies that evaluated the effect of chronic smoking on INR control found equivalent control in smokers and non-smokers.

The mechanisms by which cigarette smoking interacts with warfarin metabolism is by causing enhanced drug clearance through induction of hepatic cytochrome P-450 activity by polycyclic aromatic hydrocarbons in cigarette smoke. Nicotine itself is not thought to alter warfarin metabolism.

The interaction between excess alcohol use and warfarin anticoagulation was evaluated in a case-control study that compared alcohol use in 265 individuals receiving warfarin who had major bleeding with 305 controls from the same cohort receiving warfarin who did not have major bleeding. The risk of major bleeding was increased with moderate to severe alcohol use and with heavy episodic drinking.

Mechanism by which alcohol use interacts with warfarin anticoagulation are many, and the contribution of various factors depends greatly on the amount of intake and the severity of associated liver disease. Excess alcohol consumption may interfere with warfarin metabolism. Severe liver disease may also be associated with coagulopathy, thrombocytopenia, and/or gastrointestinal varices, all of which increase bleeding risk independent of effects on warfarin metabolism.

Comorbidities

The major comorbidities that affect anticoagulation control are hepatic disease, renal dysfunction, and heart failure. In addition, other comorbidities such as metastatic cancer, diabetes, or uncontrolled hyperthyroidism may also play a role.

The liver is the predominant site of warfarin metabolism. It is also the source of the majority of coagulation factors. Thus, liver disease can affect warfarin dosage, INR control, and coagulation in general. Importantly, individuals with severe liver disease are not “auto-anticoagulated,” because they often have a combination of abnormalities that both impair hemostasis and increase thrombotic risk.

Warfarin undergoes partial excretion in the kidney. Patients with kidney disease can receive warfarin, and management is generally similar to the population without renal impairment; however, dose requirement may be lower.

Heart failure has been shown to interfere with INR stabilization.

Acute illnesses may alter anticoagulation through effects on vitamin K intake, VKA metabolism, and medication interactions, especially infections and gastrointestinal illnesses.

Genetic Factors

Genetic polymorphisms have been implicated in altered sensitivity to warfarin and other vitamin K antagonists.

Factors Contributing to Drug-Induced Diseases

May 28, 2015 Adverse Drug Reactions, Pharmacoeconomics, Pharmacogenetics, Pharmacokinetics, Pharmacology No comments , ,

470px-Emblem_of_the_United_NationsPharmacokinetic and pharmacodynamic factors

Toxic or exaggerated responses to normal drug doses can lead to drug-induced diseases and other adverse events for two reasons. First, excessively high concentrations of unbound drug or metabolite at the site of action may occur as a result of unusual pharmacokinetics (absorption, distribution, metabolism, excretion) of the drug or metabolites. Second, response to a given concentration of unbound drug or metabolite at the site of action (pharmacodynamics) may be exaggerated or unusual because of many factors, including changes in the number and/or binding affinity of target receptors as well as alternations in signal-transduction pathways. The pharmacokinetic and pharmacodynamic behaviours of drug may be influenced by factors related to concurrent diseases, physiologic status, concomitantly administered drugs or food, lifestyle, and genetic variability.

Concurrent Diseases

The effects of disease on pharmacokinetics have been extensively reviewed. For drugs or drug metabolites that are highly dependent on the kidney or liver for elimination, usual does produce higher-than-normal serum drug concentrations in patients with kidney or liver disease respectively. This results in an exaggerated response, particularly for drugs with narrow therapeutic index.

Cardiovascular diseases, such as acute myocardial infarction or heart failure, may reduce hepatic and/or renal blood flow and decrease the elimination of drugs that are normally highly extracted by the liver. Serum concentrations and response are increased if such drugs are administered intravenously.

Hypothyroidism may be associated with a decrease in both hepatic and renal drug clearances.

Many diseases can also decrease the serum protein binding of drugs. For example, kidney and liver diseases have been associated with decreased albumin binding of some drugs. These changes generally do not result in an altered clinical response, however, because the concentration of unbound drug is unaffected. Nevertheless, misinterpretation of total serum drug concentrations of highly protein-bound drugs that are routinely monitored may lead a clinician to inappropriately increase the dose, thus resulting in toxicity.

Concurrent diseases may also be associated with enhanced pharmacodynamic responses to drugs for a variety of reasons. For example, the incidence of skin rashes and serious dermatologic adverse effects such as Stevens-Johnson syndrome is increased in patients with human immunodeficiency virus (HIV) infection who are taking trimethoprim-sulfamethoxazole. Patients with cytomegalovirus infection who are taking ampicillin are also at increased risk for skin rashes. Although the precise mechanisms are unkown, desensitisation procedures generally reduce the severity of the adverse effect if rechallenge with the drug is necessary.

Physiologic Conditions

The pharmacokinetics of a drug may be affect by age, pregnancy, and sex. In general, the rate of elimination of a drug is impaired in premature newborns, increases in early childhood to more efficient rates than those in adults, and then progressively declines with advancing age. In addition, early patients may also suffer from decreased mental status or diminished physical function. When these physiologic conditions are compounded by decreased elimination of some drugs, the patient is more susceptible to drug-induced falls or physical injury (i.e., benzodiazepines). A variety of physiologic changes during pregnancy may affect the pharmacokinetics of drugs, but no consistent patterns have been identified. The higher serum concentrations of digoxin reported in pregnant patients may be caused by increased bioavailability resulting from decreased gastric emptying time. Women have a higher risk of drug-induced adverse events as compared with men, which has been attributed to the fact that they take more drugs than men, have lower activity of drug-metabolising enzymes, and have estrogen-related effects on drug receptors, as in the case of drug-induced torsades de pointes.

DDIs

Drug-drug interactions may cause altered pharmacokinetics (bioavailability, distribution, clearance) or altered pharmacodynamics by additive or antagonistic effects. Such interactions have been extensively reviewed and are more often a predictable and preventable cause of morbidity and mortality. By far the most frequent contributing factors to drug-induced disease resulting from drug interactions are those that affect bioavailability and drug elimination. A large number of important interactions occur in the liver and gastrointestinal tract because of decreases in the rate of a drug’s metabolism or transport caused by other drugs that are inhibitors of these system.

P-glycoprotein, a transporter protein expressed on the luminal surfaces of intestinal epithelial cells, biliary hepatocytes, and renal tubular cells, transports drug from within the cell to the intestine, bile, or urine, respectively. Inhibition of P-glycoprotein can therefore increase the bioavailability and decrease renal and biliary excretion of affected drugs, thus increasing their serum concentrations. Other interaction mechanisms that may result in reduced renal excretion include reduction of the glomerular filtration rate and increased reabsorption of drug into blood from the renal tubule.

Drugs that displace other drugs from serum protein-binding sites without affecting their metabolism cause only transient increases in unbound drug concentration, and hence do not generally result in adverse effects. Displacement of drugs such as warfarin (long half-life, small volume of distribution, and narrow therapeutic range) can be clinical important, however, and necessitate a temporary reduction in the dose of the affected drug.

Serious drug-drug interactions may also occur when drugs produce additive effects through different mechanisms. Examples include: 1.the combined blood-pressure-lowering effects of calcium channel-blockers and β-blockers; 2.the increased risk of gastrointestinal bleeding resulting from non steroidal anti-inflammatory drug (NSAID)-omdiced gastric erosion in patients taking warfarin; and 3.exaggerated cyclic guanine monophosphate (cGMP)-mediated smooth-muscle relaxation caused by the combination of sildenafil (which inhibits cGMP degradation) and nitrates (which increase GMP production), leading to potentially serious hypotension. An example of a drug-drug interaction involving active or toxic metabolites is the potentiation of acetaminophen hepatotoxicity in patients receiving enzyme inducer such as rifampin, presumably by increasing the formation of toxic acetaminophen metabolites.

Drug-Food Interactions

Drug-food interactions have been extensively reviewed. One of the most serious drug-food interactions occurs with first generation nonselective monoamine oxidase inhibitors and tyramine, an amino acid found in aged or fermented foods and beverages. The suppressed metabolism of large amounts of tyramine may result in hypertensive-crisis, the so-called cheese reaction (tyramine, dopamine, norepinephrine and epinephrine are all monoamines). A large dietary intake of tyramine (or a dietary intake of tyramine while taking MAO inhibitors) can cause the tyramine pressor response, which is defined as an increase in systolic blood pressure of 30 mmHg or more. The displacement of norepinephrine (noradrenaline) from neuronal storage vesicles by acute tyramine ingestion is thought to cause the vasoconstrictionand increased heart rate and blood pressure of the pressor response. In severe cases, adrenergic crisis can occur (from wiki at http://en.wikipedia.org/wiki/Tyramine).

Components of grapefruit juice are known to suppress the presystemic elimination of certain drugs that are either metabolised in the intestinal wall by the cytochrome P-450 isozyme CYP3A4, are substrates for P-glycoprotein, or both, resulting in increased bioavailability. Drugs for which bioavailability increases dramatically when taken with grapefruit juice include lovastatin, simvastatin, buspirone, and amiodarone.

In general, an increase in drug bioavailability caused by food intake is not a problem if doses throughout the course of treatment are taken consistently at a fixed time relative to a meal. However, a clinically important drug-food interaction was reported for a particular once-daily theophylline product, which has since been reformulated. When taken with high-fat meals, a sudden, rapid release of a large amount of theophylline occurred, leading to excessively hight serum theophylline concentrations.

The salt, protein, or vitamin content of the diet also may affect the renal excretion of drugs. For example, a patient taking lithium who initiates a low-salt diet for treatment of hypertension or heart failure excretes less lithium, resulting in higher serum lithium concentrations and potential toxicity, given this drug’s narrow therapeutic range. A low-protein diet is associated with decreased renal clearance of oxypurinol, apparently through enhanced reabsorption efficiency by the uric acid transporter system.

Lifestyle Factors

Alcohol and caffeine consumption can affect both the pharmacokinetics and the pharmacodynamics of other drugs, leading to serious drug-induced diseases. There are many examples of alcohol exaggerating the central nervous system depressant effects of drugs, including benzodiazepines, phenothiazines, tricyclic antidepressants, opiates, and anthistamines. Caffeine has an additive and potentially dangerous stimulant effect when taken with ephedrine in herbal weight-loss and athletic performance-enhancing supplements.

Genetic Variability

As a result of the rapidly evolving field of pharmacogenomics, interindividual differences in drug related toxicity and therapeutic response are not always considered to be “idiosyncratic” responses. Rther, it is widely recognized that genetic makeup is responsible for a significant portion of drug-induced diseases. Many genes that encode metabolic enzymes or drug transporters are polymorphic, meaning that some groups of patients with certain gene variants have relatively inactive enzymes or transporters, while others have unusually active forms. In addition, the proportion of patients with active or inactive forms may differ among racial groups.

Polymorphisms in metabolising enzymes have been extensively reviewed. Patients with low N-acetyltransferase activities, known as “slow acetylators,” are more likely to suffer from peripheral nerve damage when administered standard isoniazid doses as compared with fast acetylators. Likewise, slow acetylators of hydrazine are more likely to suffer from hydaralazine-induced lupus erythematosus. At least four of the major cytochrome P-450 isozymes (CYP2A6, CYP2C9, CYP2C19, CYP2D6) responsible for oxidative drug metabolism are polymorphic in nature. Patients with gene variants that produce low-activity enzymes for the metabolism of warfarin experience a higher risk of serious bleeding events. Severe and potentially fatal hematologic toxicity occurs in the small percentage of patients receiving azathioprine or mercaptopurine who have a genetic deficiency in the thiopurine methyltransferase enzyme.

Polymorphisms in receptors, ion channels, or other proteins involved in drug response also occur. These result in widely variable pharmacodynamic responses among patients despite similar concentrations of the drug at the site of action. Examples include the polymorphisms in: 1.dopamine receptors, which affect the risk of drug induced tardive dyskinesia; 2.skeletal-muscle ryanodine receptors, which affect the risk of anesthesia-induced malignant hyperthermia; 3.potassium or sodium channels, which affect the risk of potentially fatal tornadoes de points when certain antiarrhythmic drugs are administered; 4.glucose-6-phosphate dehydrogenase, which if deficient, leads to red-cell homeless in patients who take drugs with a high redox potential, such as aspirin, nitrofurantoin, sulfonamides, and quinidine; and 5.the major histocompatibility complex, which mediates hypersensitivity reactions to drugs such as abacavir and nevirapine.


Adherence To Prescribed Therapy

Noncompliance implies that the patient is intentionally or willfully not following directions for medication use, which may or may not be the case. For this reason, some prefer the term non adherence, because it places no blame on either the patient or the health care professional. Common causes of non adherence are listed in Table 1. Regardless of the cause, non adherence can lead to drug-induced disease. Patients may take more or less drug than prescribed or recommended, modify medications in an inappropriate fashion (e.g., crush a sustained-release tablet), or continue to take a prescribed drug even though the underlying medical condition for which the drug was originally prescribed has resolved. Any of these actions can put patients at increased risk for drug-induced disease.

Screen Shot 2015-05-28 at 5.01.13 PMIt is estimated that the prevalence of medication non adherence is 40% to 70%. McDonnell and Jacobs analyzed the cause for hospital admissions from preventable adverse drug reactions. Of 158 drug-related hospital admissions over an 1-month period, patient non adherence with a prescribed medication regimen was the identified cause of 33% of these admissions. Unfortunately, health care providers typically are unable to identify patients at risk for non adherence. Patient age, sex, race, intelligence, and educational background have not been shown to be predictive of adherence or non adherence to a prescribed drug regimen.

Until the most effective strategy or strategies to improve adherence can be identified, it is recommended that healthcare professionals keep drug regimens simple, provide clear and complete instructions, encourage medication adherence by scheduling regular appointments, respond clearly and promptly to patients’ questions and concerns, and reinforce adherence with the prescribed regimen at every opportunity.


Medication Errors

Medication errors contribute significantly to the problem of drug-induced disease. These errors can include a variety of problems involving any step in the drug-use process. In an study, 67% of hospital admissions caused by adverse drug reactions were attributed to inadequate patient monitoring (e.g., failure to order appropriate laboratory tests to monitor drug response or failure to respond appropriately to abnormal laboratory-test results) and 51% were caused by inappropriate drug doses.

Fortunately, not all medication errors result in clinically significant problems. In one study of hospitalized patients, only 7 to 100 medication errors were thought to be serious enough to cause harm (all adverse effects are harm and might not be preventable, whereas all medication errors are preventable and do not necessarily cause harm). However, in a view of the overall frequency of medication errors that occur each year, a 7% proportion of serious medication errors may result in many affected lives.

Medication errors and drug-induced diseases may result from the limitations of an individual practitioner, from problems latent in the system or setting in which the practitioner operates, or from a combination of the above. In most cases, medication errors are caused by system-related problems including the absence of redundancies and system defenses needed to detect and counter an error in an individual’s judgment before the medication reaches the patient.