Erythrocytes smaller, on average, then 6.2 microm in diameter.

Is associated with decreased in hemoglobin synthesis.

May be related to a deficiency of iron, and impaired globulin synthesis, or a mitochondrial abnormality affecting heme synthesis.

May be associated with malabsorption syndromes, iron deficiency anemia, hemoglobinopathies.

Microcytic anemias are due to defects in hemoglobin synthesis because of iron deficiency, inadequate delivery of iron the anemia of chronic inflammation, impaired production of heme by sideroblastic anemia, or defective synthesis of the globin component of hemoglobin in thalassemia.

Chronic inflammation and sideroblastic anemia almost never present within MCV below 70 fl.

The mean corpuscular volume in iron deficiency rarely falls below 80 fl until the hematocrit is well below 30 and the red blood cell count is less than 4 million.

In thalassemia the MCV is disproportionately low for the degree of anemia.

Microcytosis develops late with iron deficiency when hemoglobin synthesis is markedly impaired and the anemia is severe.

In thalassemia defective hemoglobin synthesis is present throughout life.

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