The Red Cell Distribution Width (RDW) is a measurement found on a standard Complete Blood Count (CBC) test. This parameter quantifies the variation in the size and volume of your red blood cells, also known as erythrocytes. Although the term includes “width,” it refers to the statistical spread of their volumes, not the physical diameter of the cells. The RDW provides early insight into red blood cell production health, often flagging issues before other blood markers change.
What RDW Measures
The RDW test measures the degree of difference in size among the red blood cells circulating in your bloodstream. In healthy individuals, red blood cells are relatively uniform in size. A significant divergence from this uniformity is referred to as anisocytosis.
The RDW calculation is derived from cell volume measurements taken by the laboratory analyzer. RDW is commonly reported as a percentage, known as RDW-CV (Coefficient of Variation). This value is mathematically determined by dividing the standard deviation of the red cell volume by the Mean Corpuscular Volume (MCV). A different measurement, RDW-SD (Standard Deviation), measures the actual width of the red blood cell size distribution curve.
Measuring this size variation is significant because red blood cells are responsible for transporting oxygen throughout the body via the protein hemoglobin. When the body’s ability to produce healthy, uniform red blood cells is impaired, the resulting cells may be abnormally large or small. For example, if the bone marrow is struggling to produce cells due to a nutrient deficiency, it may release a mix of new, irregularly sized cells alongside older, existing cells. This mixed population of small and large cells is precisely what causes the RDW value to increase.
Interpreting RDW Results
An RDW result is usually presented as a percentage, which is compared against a laboratory’s established reference range. While ranges can vary slightly between labs, the typical normal range for RDW-CV in adults is generally between 11.5% and 15.0%. A result within this range suggests that the red blood cells are largely consistent in size and volume.
A high RDW means that there is a greater-than-normal variation in the sizes of the red blood cells in the sample, indicating anisocytosis. This high value signifies that the patient has a mixture of cells that are much smaller or much larger than the average. This variation can be an early indicator of a problem with red blood cell production or survival.
A low RDW means that the red blood cells are extremely uniform in size. This result is rare and is generally not considered clinically significant on its own, nor is it associated with any specific type of anemia or disease state.
The RDW result is rarely interpreted in isolation; its diagnostic power is unlocked when viewed alongside the Mean Corpuscular Volume (MCV). The MCV indicates the average size of the red blood cells, while the RDW provides information about the spread of those sizes. By combining these two metrics, healthcare providers can narrow down the potential causes of an abnormality. For instance, a high RDW paired with a low MCV suggests a very different set of conditions than a high RDW paired with a high MCV.
Clinical Relevance of Abnormal RDW
An elevated RDW is a common finding in many types of anemia, helping to differentiate one form of the condition from another. For example, iron deficiency anemia often presents with a high RDW and a low MCV, meaning the cells are smaller than average but highly variable in size. This pattern occurs because the body is trying to compensate for low iron by producing new, smaller cells, which mix with the existing, normal-sized cells.
Conversely, a high RDW combined with a high MCV often suggests a deficiency in Vitamin B12 or folate. These nutritional deficiencies impair DNA synthesis, which leads to the production of fewer, but abnormally large, red blood cells. The high RDW in this case reflects the presence of these new, large cells alongside older, possibly normal-sized cells.
The RDW is also a tool for distinguishing between iron deficiency anemia and conditions like thalassemia minor, which can both cause small red blood cells (low MCV). Thalassemia minor, an inherited blood disorder, typically results in a low MCV but a normal RDW because all the red cells are uniformly small. An elevated RDW, therefore, often steers the diagnosis toward iron deficiency when a low MCV is present.
Beyond anemia, an elevated RDW can signal active red blood cell destruction or an issue with bone marrow function. An increased RDW is also associated with a higher risk of adverse outcomes in other conditions, including heart disease, kidney disease, and chronic liver disease. In these scenarios, the RDW elevation often reflects systemic inflammation or poor nutritional status.