The Red Cell Distribution Width (RDW) is a measurement reported as part of a standard Complete Blood Count (CBC) test. This value specifically quantifies the variation in size and volume of a patient’s red blood cells. An elevated RDW value suggests that the red blood cells present in the bloodstream are not uniform, a condition known as anisocytosis.
What RDW Measures and Reference Ranges
Healthy red blood cells are typically uniform in size, measuring between 6.2 and 8.2 micrometers in diameter. When the body’s machinery for producing these cells is disrupted, the resulting cells can be a mixture of unusually small, normal, and unusually large cells. A higher percentage value reflects a wider distribution of cell sizes.
The standard reference range for RDW typically falls between 11.5% and 14.5%, although this can vary slightly between laboratories. A result within this range indicates that the red blood cells are relatively consistent in size. While a slightly elevated RDW may indicate an early or mild condition, values that are significantly high, such as those exceeding 18% or 20%, are generally associated with more pronounced underlying issues.
Underlying Conditions That Elevate RDW
An elevated RDW is a common finding in many conditions that interfere with the bone marrow’s ability to produce healthy, uniform red blood cells. The most frequent causes involve nutritional deficiencies, which directly impact the materials needed for proper cell maturation. For example, iron deficiency anemia often leads to the production of small, pale red blood cells, called microcytes. As these new microcytes enter the bloodstream alongside older, normal-sized cells, the RDW increases because of the mixed population of cell sizes.
Similarly, deficiencies in Vitamin B12 and folate disrupt DNA synthesis, causing the bone marrow to release abnormally large red blood cells, known as macrocytes. In some cases, a person may have a combined deficiency, such as both iron and folate, which results in a highly heterogeneous population of both small and large cells, leading to a significantly elevated RDW.
Conditions involving chronic inflammation can also lead to an elevated RDW by disrupting the regulatory signals for red cell production. Chronic kidney disease, for instance, can impair the production of erythropoietin, a hormone that stimulates red blood cell creation, leading to ineffective cell formation and size variation. Similarly, chronic liver disease and certain heart conditions are associated with higher RDW values, reflecting a stress response or systemic disruption to the body’s cell-producing environment.
Genetic disorders that inherently affect red blood cell structure and size are another distinct cause of RDW elevation. Conditions like Thalassemia, which involves defects in hemoglobin synthesis, and Sickle Cell Trait can cause red blood cells to be produced in various irregular shapes and sizes. This inherent morphological variation in the red cell population directly results in an elevated RDW.
Diagnostic Interpretation and Follow-Up Testing
Interpreting a high RDW is rarely done in isolation; it functions primarily as a warning sign that guides further investigation. Healthcare providers routinely use the RDW in conjunction with the Mean Corpuscular Volume (MCV), which measures the average size of the red blood cells. By comparing these two values, a clinician can narrow down the potential categories of the underlying disorder.
For instance, an elevated RDW combined with a low MCV strongly suggests iron deficiency anemia. Conversely, a high RDW coupled with a high MCV points toward deficiencies in Vitamin B12 or folate, where the red cells are, on average, larger than normal. A high RDW with a normal MCV can suggest a mixed deficiency or an early-stage nutritional deficiency where the average size has not yet changed significantly.
To confirm the cause suggested by the RDW and MCV pattern, specific follow-up testing is necessary. If nutritional deficiency is suspected, a doctor will typically order blood tests to measure ferritin levels, which reflect the body’s stored iron, as well as serum Vitamin B12 and folate levels. In cases where the initial blood work is inconclusive, a peripheral blood smear may be performed, which involves examining a blood sample under a microscope to visually confirm the variation in cell size and shape.