Historically, evaluation of immunocompetence has been included as a part of any discussion of protein and nutrition assessment. This is logical, since adequate and appropriate immune function is dependent in part on adequate protein status. Protein deficiency routinely results in increased risk of infection as well as altered immune and inflammatory responses. But in clinical practice, the use of this type of nutrition assessment is complicated by the presence of disease and infection, which of course also affect all components of the immune system.
Nutrition Care Indicator: Total Lymphocyte Count (TLC) When evaluating a complete blood count (CBC) and differential count, calculaltion for TLC can be completed as follows:
TLC = WBC X % lymphocytes / 100
Total lymphocyte count will be affected by presence of infection, trauma, stress, and diseaes such as cancer and HIV, as well as medications that influence the immune system (e.g., chemotherapy and corticosteroids).
Nutrition Care Indicators for Hematological Assessment
Evaluation of erythrocytes (red blod cells, or RBC) can be an important component of nutrition assessment and is key to diagnosis of all anemia types. A complete blood count includes measurement of the total number of blood cells in the volume of blood. Many types of anemias exist, including those caused by deficiencies of iron, folate, or vitamin B12 and those arising from chronic diseases such as renal failure and congestive heart failure. Anermias are diagnosed by evaluation of the complete blood count and by the microscopic evaluation of the size, shape, and color of erythrocytes.
Hemoglobin (Hgb) Hemoglobin is a protein found in erythrocytes that functions to deliver oxygen to cells and to pick up carbon dioxide for expiration by the lungs. Measurement of hemoglobin is common in diagnosis of anemias, particularly iron-deficiency anemia. Additionally, hemoglobin is decreased in some chronic diseases and protein-energy malnutrition. Even though it is commonly measured, it is not the most sensitive or the most specific of hematological assessments of nutritional status. For example, in iron deficiency, iron stores may be depleted before serum hemoglobin levels will be affected.
Hematocrit (Hct) Hematocrit is defined as the percentage of blood that is actually composed of red blood cells. Hematocrit, like hemoglobin, will be decreased only in the final stage of iron deficiency. Hematocrit is affected by other nutrient deficiencies as well as by hydration status.
Mean Corpuscular Volume (MCV) Mean corpuscular volume is a measure of the average size of an individual red blood cell. A variety of anemias are characterized by changes in RBC size; for example, MCV is reduced in iron an copper deficiencies and elevated in folic acid and vitamin B12 deficiencies.
Mean Corpuscular Hemoglobin (MCH) Mean corpuscular hemoglobin is an estimate of the amout of hemoglobin in each cell. Thi value can reflect total serum hemoglobin levels. In some situations, however, MCH remains normal while the number of red blood cells is low, resulting in low total Hgb. Abnormalities are generally specific to iron deficiency and other nutritional anemias.
Mean Corpuscular Hemoglobin Concentration (MCHC) Mean corpuscular hemoglobin concentration also estimates the amount of hemoglobin in each red blood cell, but it expresses the value as a percentage.
Ferritin Ferritin is a protein that serves as a storage form of iron; therefore, serum ferritin is an estimate of iron stores. Ferritin is a sensitive and specific measure of iron status and will be one of the first indices to change in iron deficiency.
Transferrin Saturation As discussed earlier under "Protein Assessment," transferrin is a serum protein responsible for transport of iron systemically. Each molecule of transferrin can carry two molecules of iron. Under normal conditions, approximately 30% of iron binding sites on the transferrin molecule are saturated (i.e., have iron attached). The body's requirement for iron and overall iron status will be reflected by changes in transferrin saturation. When iron status is low, transferrin is less saturated. Transferrin is calculated by using the ratio of serum levels to total iron biding capacity (TIBC). TIBC is the test used to measure the saturation ability for transferrin. TIBC is higher during iron deficiency and lower after repletion. There are numerous equations to calculate transferrin from TIBC, but, as mentioned earlier, transferrin is not the most reliable indicator of protein status due to the effect of iron status.
Protoporphyrin When there is inadeuate iron available for hemoglobin synthesis, zinc is substituted fro iron within hemoglobin. Consequently, zinc protoporphyrin (the protein transporter for zinc) levels rise during iron deficiency and are considered a sensitive measure of iron-deficiency anemia.
Serum Folate Coenzymes associated with folate are necessary for amino acid metabolism, including many one-carbon transfer reactions such as the conversion of histidine to glutamate. Folate coenzymes also play a cruical role in the synthesis of purine needed for DNA. Folate deficiency can be diagnosed when megaloblastic, macrocytic red blood cells are present and serum folate and red cell folate are decreased, while serum B12 remains within normal limits. If folate levels are inadequate for conversion of histidine to glutamate, an intermediate product, formiminoglutamate (FIGlu), is formed. Urinary levels of FIGlu are thus elevated in folate deficiency and serve as a diagnostic tool for the condition.
Serum B12 Anemia associated with B12 (cobalamin) deficiency can be diagnosed in several ways. Clinically, it will be similar to folate deficiency but can be distinguished by measuring serum B12 levels, including serum total cobalamin and serum holo-transcobalamin II (the transport protein for B12). Biomarkers of B12 include homocysteine and methylmalonic acid levels, which change early on in the development of B12 deficiency. Historically, the Schilling test allowed for determination of defective absorption (gastric vs. intestinal). In this test, B12 is given as an injection and the amount excreted in urine is measured. This allows problems with different steps of B12 absorption to be distinguished. The Schilling test is no longer used in clinical practice, though to date no other test has replaced it specific function.