The Red Cell Distribution Width (RDW) is a measurement found within a standard Complete Blood Count (CBC) test. This parameter quantifies the variation in the size of your circulating red blood cells. The RDW provides an objective numerical value that reflects the uniformity of these cells. A result outside the expected range can signal an underlying issue with the production or lifespan of red blood cells.
Defining Red Cell Distribution Width (RDW)
The RDW is a laboratory metric that measures the degree of size difference among red blood cells (erythrocytes) in a blood sample. In healthy individuals, these cells are uniform in size. The medical term for a greater-than-normal variation in cell size is anisocytosis, and a high RDW value directly correlates with increased anisocytosis.
This measurement is a statistical calculation, typically expressed as a percentage. The most common format is the RDW-Coefficient of Variation (RDW-CV). Automated blood analyzers use this calculation to reflect the width of the red cell volume distribution curve. A wider curve means a greater spread of cell sizes and, consequently, a higher RDW reading.
The consistency of red blood cell size is important because these cells transport oxygen. When red cells are significantly varied in size, their ability to navigate the microvascular network and deliver oxygen effectively is diminished. Therefore, the RDW provides insight into the functional quality of the red cell population. Problems in the bone marrow, where red cells are produced, often manifest as a high RDW.
Interpreting RDW Results
A high RDW indicates a heterogeneous population of red blood cells circulating in the bloodstream. While the normal range can vary slightly depending on the laboratory equipment used, a typical reference interval for RDW-CV is generally between 11.5% and 15.4%. A result exceeding this range is considered elevated and warrants further clinical investigation.
A high RDW is a sensitive, though non-specific, marker of underlying pathology, sometimes rising even before other red cell parameters change. The RDW is rarely interpreted in isolation and is considered alongside other CBC parameters, most notably the Mean Corpuscular Volume (MCV).
Conversely, a low RDW indicates that the red blood cells are highly uniform in size. This finding is typically not associated with any specific disease state or type of anemia, and is usually not considered clinically significant.
Underlying Causes of an Elevated RDW
An elevated RDW is most commonly associated with processes that disrupt the uniform production of red blood cells in the bone marrow. The majority of causes relate to various types of anemia, which are often differentiated using the RDW in combination with the MCV. Nutritional deficiencies are a primary driver of anisocytosis.
A high RDW with a low MCV (varied and small cells) is highly suggestive of iron deficiency anemia. When iron stores are low, the bone marrow produces smaller cells that are mixed with older, normal-sized cells. This creates two distinct populations, leading to a significant increase in the RDW.
In contrast, a high RDW coupled with a high MCV (varied and larger-than-average cells) points toward deficiencies in vitamin B12 or folate. A lack of these nutrients, which are necessary for proper DNA synthesis, results in the production of abnormally large, immature red blood cells (megaloblasts). The mixture of these large cells with existing cells causes the RDW to rise. If a patient has multiple deficiencies, such as both iron and B12 deficiency, the resulting mixed population of small and large cells will lead to a markedly high RDW.
The RDW also helps distinguish nutritional anemias from inherited disorders like thalassemia. In some forms of thalassemia, red blood cells are small (low MCV), but the RDW remains normal because the entire cell population is uniformly small. Therefore, a normal RDW with a low MCV helps separate thalassemia from iron deficiency anemia, which typically shows a high RDW.
Beyond nutritional causes, chronic inflammatory conditions can also lead to an elevated RDW. Conditions such as heart disease, chronic kidney disease, and autoimmune disorders involve ongoing inflammation and oxidative stress. This systemic stress interferes with the normal maturation and lifespan of red cells, resulting in a less uniform population and a higher RDW. High RDW values in this context are considered a general marker of systemic dysfunction.