microcytosis

Microcytosis is a hematologic finding characterized by red blood cells that are smaller than normal, reflected by a mean corpuscular volume (MCV) below the lower limit of the reference range (typically under 80 fL in adults). It can occur with or without anemia and is usually discovered on a complete blood count performed for unrelated reasons or during evaluation of fatigue or pallor.

The most common causes are disorders of hemoglobin synthesis. Iron deficiency anemia is the leading cause worldwide and often presents with microcytosis and hypochromia; anemia of chronic disease or inflammation can also produce microcytosis, sometimes with normal iron stores. Thalassemias (alpha or beta) cause microcytosis due to defective globin chain production and may show normal or elevated RBC counts with low MCV. Sideroblastic anemia and lead poisoning are other causes where impaired heme synthesis leads to smaller red cells. Less commonly, combined deficiencies or rare genetic conditions can contribute. The presence of microcytosis with normal ferritin and inflammation markers may raise suspicion for thalassemia trait, whereas markedly low ferritin supports iron deficiency.

Diagnosis involves confirming low MCV, examining the peripheral smear (often hypochromia, sometimes target cells), and conducting iron studies (serum ferritin, iron, TIBC, transferrin saturation). Hemoglobin electrophoresis or genetic testing may be used to identify thalassemias; RDW can help differentiate iron deficiency (often elevated) from thalassemia (often normal). Treatment targets the underlying cause: iron supplementation for iron deficiency, management of chronic disease, or genetic counseling for thalassemia.