Evaluation of microcytosis.

Microcytosis is typically an incidental finding in asymptomatic patients who received a complete blood count for other reasons. The condition is defined as a mean corpuscular volume of less than 80 µm3 (80 fL) in adults. The most common causes of microcytosis are iron deficiency anemia and thalassemia trait. Other diagnoses to consider include anemia of chronic disease, lead toxicity, and sideroblastic anemia. Serum ferritin measurement is the first laboratory test recommended in the evaluation of microcytosis. Low ferritin levels suggest iron deficiency. Once a presumptive diagnosis of iron deficiency anemia has been made, an underlying source for the deficiency should be determined. Iron deficiency anemia in adults is presumed to be caused by blood loss; the most common source of bleeding is the gastrointestinal tract. The possibility of gastrointestinal malignancy must be considered. If the serum ferritin level is not initially low, further evaluation should include total iron-binding capacity, transferrin saturation level, serum iron level, and possibly hemoglobin electrophoresis. Anemia of chronic disease is suggested with low iron levels and decreased total iron-binding capacity. Patients with beta-thalassemia trait usually have elevated levels of hemoglobin A2.