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Macrocytosis is a condition where red blood cells are larger than normal. [1] These enlarged cells, also known as macrocytes, are defined by a mean corpuscular volume (MCV) that exceeds the upper reference range established by the laboratory and hematology analyzer (usually >110 fL). [2]
The mean corpuscular volume, or mean cell volume (MCV), is a measure of the average volume of a red blood corpuscle (or red blood cell). The measure is obtained by multiplying a volume of blood by the proportion of blood that is cellular (the hematocrit), and dividing that product by the number of erythrocytes (red blood cells) in that volume.
The normal mean corpuscular volume (abbreviated to MCV on full blood count results, and also known as mean cell volume) is approximately 80–100 fL. When the MCV is <80 fL, the red cells are described as microcytic and when >100 fL, macrocytic (the latter occurs in macrocytic anemia). The MCV is the average red blood cell size.
Macrocytic anemias have several causes but with the implementation of folic acid fortification in North America, folate deficiency has become a rare cause of megaloblastic macrocytic anemia in that part of the world. [1] In this region, Vitamin B 12 deficiency is a far more common cause of megaloblastic macrocytic anemia. [1]
Folate and vitamin B 12 deficiency anemia usually presents with high RDW and high MCV. Mixed-deficiency (iron + B 12 or folate) anemia usually presents with high RDW and variable MCV. Recent hemorrhages typically present with high RDW and normal MCV. A false high RDW reading can occur if EDTA anticoagulated blood is used instead of citrated blood.
A diagnosis of PA first requires demonstration of megaloblastic anemia by conducting a full blood count and blood smear, which evaluates the mean corpuscular volume (MCV), as well the mean corpuscular hemoglobin concentration (MCHC). [50] PA is identified with a high MCV (macrocytic anemia) and a normal MCHC (normochromic anemia). [54]
For determination of cause, further patient history, testing, and empirical therapy may be clinically indicated. A measurement of methylmalonic acid (methylmalonate) can provide an indirect method for partially differentiating Vitamin B 12 and folate deficiencies. The level of methylmalonic acid is not elevated in folic acid deficiency.
The mean corpuscular volume or MCV is usually normal or low for congenital causes of sideroblastic anemia but normal or high for acquired forms. With lead poisoning , see coarse basophilic stippling of red blood cells on peripheral blood smear