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What Clinical Possibilities Should I Consider in a Patient With an Increased MCV? What Preanalytic Variables May Increase the MCV?

When a patient presents with a macrocytic blood picture, there are several clinical possibilities to consider. The most obvious reason for an increased MCV is a patient with a megaloblastic process. Supporting laboratory data for this possibility would include a pancy-topenia, a reticulocytopenia, increased LDH, and a peripheral smear with macro-ovalocytes, hyperseg-mented neutrophils, and other poikilocytes. Follow-up testing should initially include an assessment of the vitamin B12 and folic acid levels, as well as testing for intrinsic factor antibodies.

A second patient population to consider when assessing a macrocytic anemia would be those who have liver disease, alcoholic cirrhosis, hypothyroidism, or chemotherapy. These patients would NOT show a pancytopenia but would show a moderate anemia with slightly increased MCV with round microcytes and perhaps siderocytes. A thorough review of the patient history should give insights into the nature of the macrocytic anemia. An often forgotten but fairly consistent reason for a slightly increased MCV is regenerative bone marrow. Patients who have inherited blood disorders such as sickle cell anemia, thalassemia major, or other hemolytic processes are transfused on a regular basis as part of their disease management. Not only do the transfused cells lend some size variation to their clinical process, but also the chronic anemia in these patients leads to a premature release of reticulocytes, which are immature cells that are larger than normal red cells. When reticulocytes are stained with Wright's stain, polychromatophilic macrocytes appear in the peripheral smear. In a peripheral smear with increased polychromasia, a slightly macrocytic blood picture is often seen. A simple assessment for the reticulocytes will show an increased value, which is contributory to the source of the increased MCV.

The MCV is a highly stable parameter, yet several preanalytic variables can alter the MCV If a sample fails a delta check as a result of a rise in MCV, several considerations are in order. Sample contamination may increase the red cell size, especially if the sample is drawn through an intravenous line or line that has been flushed with anticoagulant. Another condition capable of raising the MCV is high glucose volume either as a result of a diabetic episode or coma or as a result of blood drawn through the intravenous glucose infusion line. A quick check of the glucose level in the sample should reveal the source of the erratic MCV.

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