Mean corpuscular volume is the average volume of a red blood cell. A mean corpuscular volume greater than 100 fL (femtoliters) indicates a macrocytic anemia.
Vitamin B12 is found in meat and dairy. Since stores remain in the body for years, deficiency is rare, but can occur in patients with celiac sprue, pernicious anemia, gastric bypass surgery, or long-term vegans for example. B12 deficiency is associated with increased homocysteine and methylmalonic acid levels.
Folate is found in a variety of foods such as grains, fruits, vegetables, and poultry, and folate stores remain in the body for about 4 months. Pregnant woman should be supplemented with folate as deficiency during pregnancy is associated with neural tube defects. Folate deficiency is associated with increased homocysteine levels but normal amounts of methylmalonic acid.
Orotic aciduria is an autosomal recessive disease involving deficiency of an enzyme in the de novo pyrimidine synthesis pathway. These patients have increased orotic acid in the urine, and may have a megaloblastic anemia that is not corrected with folate or B12 supplementation. Treatment involves oral uridine supplements.
Alcohol is a direct toxin to the bone marrow, damaging RBC precursor cells and causing a non-megaloblastic macrocytic anemia. Chronic alcoholism also damages the liver, further complicating the anemia.
Liver disease involving cholesterol esterification deficiency affects RBC membranes by making them contain excess cholesterol, leading to production and appearance of macrocytic target cells.
When RBCs are created, they are larger than the average RBC. In states of rapid cell production and proliferation, the average MCV is shifted, suggestive of a macrocytic anemia. However, as the newly created RBCs age, their cell size shrinks, and the MCV adjusts to a normocytic range.
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