Anisocytosis is a laboratory finding indicating that a patient’s red blood cells (RBCs) are of unequal or varied sizes, deviating from the uniform size typically found in healthy blood samples. The term is derived from the Greek, meaning “unequal cells.” Its presence suggests an underlying issue is disrupting the body’s normal process of blood cell production, often affecting the cells’ ability to efficiently carry oxygen. When detected during routine screening, anisocytosis prompts further investigation to determine the specific cause of the size variation.
Defining Anisocytosis and How It is Measured
The presence of anisocytosis is quantitatively measured in a standard Complete Blood Count (CBC) test using the Red Cell Distribution Width (RDW). The RDW value reflects the degree of variation in the volume of the circulating red blood cells. A normal RDW indicates uniform cell size, while an elevated RDW confirms anisocytosis.
The RDW is calculated by automated blood analyzers based on the coefficient of variation of the red blood cell volume and the Mean Corpuscular Volume (MCV). This provides an objective assessment of the spread of cell sizes. An RDW result outside the normal reference range, typically between 11% and 15% in adults, indicates a wider range of cell sizes present than expected.
While the RDW provides a precise numerical value, anisocytosis can also be visually confirmed using a peripheral blood smear. A laboratory technician or hematologist examines stained blood cells under a microscope to assess the differing sizes. This visual inspection complements the automated RDW measurement and can also detect poikilocytosis, a related condition involving abnormal cell shape.
Primary Causes of Variation in Red Blood Cell Size
Anisocytosis arises from conditions that interfere with the bone marrow’s ability to produce mature, uniformly sized red blood cells, most commonly various types of anemia. A frequent cause is iron deficiency anemia, resulting from insufficient iron needed to synthesize hemoglobin. In the initial stages of iron depletion, the body releases a mixture of normal-sized cells alongside newer, smaller cells, which elevates the RDW.
Megaloblastic anemias, typically caused by deficiencies in Vitamin B12 or folate, present a different mechanism. These nutrients are necessary for DNA synthesis, and their lack causes red blood cell precursors to grow abnormally large before dividing. This results in the release of oversized cells, known as macrocytes, which circulate alongside normal cells, contributing to size variability.
Anisocytosis is also associated with chronic disease states, such as liver disease, kidney disease, and chronic inflammatory disorders. These conditions disrupt the bone marrow’s environment and signaling pathways, impairing red blood cell production, sometimes affecting the hormone erythropoietin. Furthermore, certain bone marrow disorders, including myelodysplastic syndromes, directly impair the production line, causing the release of defective cells highly variable in size.
Categorizing Red Blood Cell Size Variation
The classification of anisocytosis is closely tied to the Mean Corpuscular Volume (MCV), which measures the average size of the red blood cells. By combining the RDW (variation) and the MCV (average size), clinicians can narrow down the potential underlying causes and organize the diagnosis based on the cells’ morphological appearance.
When anisocytosis is present with a low MCV, it is termed microcytic anisocytosis, meaning the average cell size is smaller than normal despite the wide range. This pattern characterizes iron deficiency anemia and some forms of thalassemia. Conversely, anisocytosis accompanying a high MCV is known as macrocytic anisocytosis, indicating cells are, on average, larger than normal while still showing size variation.
Macrocytic anisocytosis is commonly observed in anemias resulting from B12 or folate deficiencies. A third pattern is dimorphic or mixed anisocytosis, which occurs when two distinct populations of red blood cells circulate simultaneously. This mixed picture may be seen, for example, in a patient being treated for one deficiency who then develops another, or following a blood transfusion where donated cells differ in size from the recipient’s cells.
Addressing the Underlying Condition
Anisocytosis is a diagnostic clue, not a disease itself. Management focuses on identifying and treating the underlying medical condition. Treatment protocols are specific to the root cause, often uncovered by further testing, which includes measuring levels of iron, ferritin, B12, and folate. For nutritional deficiencies, treatment is straightforward, involving oral or injectable supplementation to restore adequate nutrient levels.
Patients with iron deficiency anemia receive iron supplements, while those with B12 or folate deficiency receive appropriate vitamin therapy. If anisocytosis stems from more severe conditions, such as certain genetic disorders or bone marrow failure, management involves intensive therapies. These advanced treatments can include regular blood transfusions or, in rare instances, a bone marrow transplant to correct the fundamental production issue.
Following treatment initiation, the RDW is frequently monitored as an objective measure to track the bone marrow’s response. A decrease in the RDW toward the normal range indicates the bone marrow is successfully producing a new, uniformly sized population of red blood cells. Continued follow-up ensures the underlying condition is resolved and the blood’s oxygen-carrying capacity is restored.