anisocytosis

Anisocytosis is a laboratory and clinical term used to describe a noticeable variation in red blood cell (RBC) sizes. It is typically assessed on a peripheral blood smear or quantified by the red cell distribution width (RDW) on a complete blood count. The term describes size heterogeneity of RBCs rather than a specific disease.

Causes and patterns: Anisocytosis can arise from mixed populations of RBCs or abnormal maturation. It is common in iron deficiency anemia and megaloblastic anemia, as well as hemolytic anemias, chronic diseases, liver disease, and after blood loss or transfusion. Severe iron deficiency often shows marked anisocytosis with microcytosis and high RDW.

Diagnosis and interpretation: RDW quantifies anisocytosis; an elevated RDW with abnormal mean corpuscular volume (MCV) helps distinguish causes. A normal RDW with isolated macrocytosis or microcytosis suggests a single population, whereas an increased RDW indicates mixed RBC populations. Peripheral smear may show microcytes, macrocytes, target cells, or stomatocytes.

Clinical significance: Anisocytosis itself is not a disease but a finding that guides diagnostic evaluation and treatment decisions. It is used alongside iron studies, vitamin B12/folate status, reticulocyte count, hemolysis tests, and inflammatory markers to determine etiology and monitor response.