The term “anisocytosis” is the medical description for variation in the size of a person’s red blood cells (RBCs). Normally, RBCs are consistently sized discs that efficiently carry oxygen. Anisocytosis means the cells are not uniform; some may be smaller, others larger, or a mixture of both. This finding is a descriptive observation, not a diagnosis itself. It serves as an important indicator that something is disrupting normal red blood cell production, prompting further investigation.
Defining Anisocytosis and the RDW Test
The presence of anisocytosis is measured through the Red Cell Distribution Width (RDW), a component of the complete blood count (CBC). The RDW test reports the range of variation in the volume and size of the red blood cells. A high RDW value indicates a significant difference in cell size, confirming anisocytosis.
The standard RDW range typically falls between 12% and 15%; results above this threshold suggest abnormal variability. RDW is a sensitive tool, often detecting changes in red blood cell production earlier than other measurements, such as the Mean Corpuscular Volume (MCV). While MCV reports the average size, RDW reveals the spread of those sizes, providing a detailed picture of red blood cell health.
Primary Categories of Anisocytosis
The variation in red blood cell size is classified based on the average cell size (MCV) to narrow down potential causes. Microcytic anisocytosis occurs when red blood cells are predominantly smaller than normal. This often correlates with conditions that impair hemoglobin incorporation, such as iron deficiency.
Macrocytic anisocytosis describes red blood cells that are mainly larger than average. This pattern points toward issues that interfere with cell division in the bone marrow, such as deficiencies in certain B vitamins. A third presentation occurs when the MCV is normal but the RDW is elevated, indicating a mix of small and large cells. This dimorphic population suggests two or more processes may be affecting red blood cell production simultaneously.
Common Underlying Causes and Associated Conditions
Anisocytosis is frequently linked to nutritional deficits that interfere with the bone marrow’s ability to manufacture uniform red blood cells. Iron deficiency anemia is a common cause, resulting in microcytic anisocytosis because developing cells lack iron to synthesize sufficient hemoglobin. The severity of the iron deficiency often correlates directly with the degree of anisocytosis observed in the RDW.
Deficiencies in Vitamin B12 or folate, crucial for DNA synthesis, lead to megaloblastic anemia and macrocytic anisocytosis. Without these vitamins, red blood cell precursors fail to divide properly, maturing into abnormally large cells. Causes include inadequate dietary intake or pernicious anemia, an autoimmune issue that prevents B12 absorption.
Anisocytosis can also be a feature of several chronic diseases that indirectly impact red blood cell production or lifespan. Conditions like chronic kidney disease and liver disease affect the body’s blood-forming processes and can lead to an elevated RDW. Chronic inflammatory states also show an association with anisocytosis, reflecting systemic disruption to the bone marrow environment.
Genetic disorders known as hemoglobinopathies, such as Thalassemia or Sickle Cell Disease, inherently cause red blood cell size variation. Thalassemia results in defective hemoglobin production, leading to microcytic cells and a high RDW. Sickle Cell Disease involves abnormal hemoglobin that causes red blood cells to deform into a sickle shape, accompanied by significant size variation. Certain treatments, including chemotherapy drugs, can also temporarily disrupt bone marrow function and lead to anisocytosis.
Next Steps After an Anisocytosis Finding
When a blood test reveals anisocytosis, the finding signals the healthcare provider to initiate a focused diagnostic search. The first step involves correlating the elevated RDW with the MCV to determine the category of anisocytosis, which directs the investigation. A peripheral blood smear is often performed to visually confirm size variation and look for abnormal cell shapes.
Subsequent blood work is ordered to pinpoint the specific deficiency or disease, focusing on nutrient levels. This typically includes serum ferritin and iron studies, plus separate tests for vitamin B12 and folate levels. Once the root cause is identified, treatment focuses on correcting that condition, involving supplementation or comprehensive management for chronic issues.