The term “aniso” in a blood test report refers to anisocytosis, which describes a population of red blood cells (RBCs) that are unequal in size. Normally, RBCs are uniform in diameter, allowing them to efficiently transport oxygen throughout the body. When a blood sample shows significant size variation, it is flagged as anisocytosis on a complete blood count (CBC) test. This finding is not a diagnosis itself, but rather points toward an underlying process affecting red blood cell production or survival.
The Quantitative Measure Red Cell Distribution Width (RDW)
Anisocytosis is measured in the laboratory by calculating the Red Cell Distribution Width (RDW), a value that quantifies the degree of size variation within the red blood cell population. The RDW measures the spread of RBC volumes around the average cell size. Automated hematology analyzers perform this calculation by analyzing thousands of individual cells and plotting their volumes on a histogram.
A normal RDW value typically falls within the range of about 11.5% to 15.4%, though the precise reference range can vary slightly between different laboratories. When the RDW is elevated above this normal range, it indicates a greater-than-normal variation in cell size, confirming the presence of significant anisocytosis. The RDW is often reported in two forms: RDW-CV (Coefficient of Variation) and RDW-SD (Standard Deviation). RDW-CV is mathematically related to the average cell volume, while RDW-SD is a direct measure of the width of the size distribution curve.
The high RDW result confirms that there is a problem with the body’s ability to produce uniformly sized red blood cells. A high RDW, indicating a wider distribution of cell sizes, is often one of the earliest laboratory signs of a developing nutritional deficiency or other underlying issue.
Categorizing Cell Size Variations
The finding of anisocytosis can be further characterized by looking at the average size of the red blood cells alongside the RDW. This combination helps narrow down the potential root cause by categorizing the types of size irregularities present in the blood.
Red blood cells that are smaller than the normal size are referred to as microcytic cells. This pattern is frequently observed when the body is unable to properly synthesize hemoglobin, often due to an insufficient iron supply. Conversely, cells that are larger than the normal size are designated as macrocytic cells. Macrocytosis typically results from defects in the maturation of red blood cell precursors in the bone marrow, commonly associated with specific vitamin deficiencies.
When a blood sample contains a mix of both significantly smaller and significantly larger cells, the population is sometimes referred to as dimorphic. This dimorphic pattern often suggests a patient has two simultaneous conditions or has recently begun treatment for one deficiency while another persists. Identifying these size categories is an important step because it directs the diagnostic process toward specific groups of underlying conditions.
Underlying Conditions Indicated by Anisocytosis
Anisocytosis is a non-specific finding, but its presence strongly suggests a disturbance in the process of erythropoiesis, which is the production of red blood cells in the bone marrow. One of the most common causes is a nutritional deficiency, which directly impairs the bone marrow’s ability to create healthy, uniform cells.
Iron deficiency, the most prevalent form of anemia globally, results in microcytic anisocytosis because the cells lack the necessary iron to form adequate hemoglobin, leading to the production of small cells. Deficiencies in Vitamin B12 or folate also commonly cause anisocytosis, but they result in macrocytic cells. These vitamins are necessary for DNA synthesis, and their lack leads to delayed cell division and the release of abnormally large, immature red cells into the bloodstream.
Different types of inherited anemias are also associated with anisocytosis, such as thalassemia, a genetic disorder affecting hemoglobin production, which typically presents with microcytic cells. Sickle cell disease also features anisocytosis due to the abnormal shape and size of the affected cells.
Conditions affecting the bone marrow directly, such as myelodysplastic syndromes, can also lead to a highly variable red cell population. Furthermore, chronic diseases affecting major organs, including liver disease and kidney disease, are frequently linked to anisocytosis. Kidney disease can impair the production of erythropoietin, a hormone that stimulates red cell production, while liver disease can interfere with nutrient storage and red cell membrane health. Because anisocytosis is a marker and not a disease, a high RDW prompts further, more targeted testing, such as measuring ferritin levels or B12/folate concentrations, to pinpoint the precise underlying cause.