Macrocytes

Macrocytes are red blood cells (erythrocytes) that are larger than the normal mean corpuscular volume (MCV). In a complete blood count, an MCV greater than 100 femtoliters indicates macrocytosis, while normal cells typically range from 80 to 100 femtoliters. The increased cell size may result from alterations in hemoglobin synthesis, membrane composition, or erythrocyte maturation.

Macrocytosis is most commonly associated with macrocytic anemias, including megaloblastic anemia caused by deficiencies of vitamin B12 or folate, and non‑megaloblastic conditions such as liver disease, alcoholism, hypothyroidism, and certain medications (e.g., retinoids, azoles, or antimetabolites). In megaloblastic anemia, ineffective DNA synthesis leads to the production of oversized, immature red cells that often have a slightly hypochromic appearance. In non‑megaloblastic macrocytosis the enlarged cells may appear more normochromic.

Diagnosis usually requires a combination of laboratory findings. Besides elevated MCV, reticulocyte count, serum levels of vitamin B12, folate, liver function tests, thyroid function tests, and a peripheral smear assessment are integral. The smear often reveals anisopoikilocytosis—marked variability in cell size and shape—as well as hypersegmented neutrophils in megaloblastic disease.

Treatment focuses on the underlying cause: vitamin B12 (cyanocobalamin) or folic acid supplementation for nutritional deficiencies, addressing liver disease or thyroid dysfunction for non‑megaloblastic causes, and modifying or discontinuing offending drugs when appropriate. Follow‑up blood counts typically show normalization of MCV within weeks of therapy.