The Red Cell Distribution Width (RDW) is a value reported as part of a routine Complete Blood Count (CBC) test, offering a specific window into the health of a person’s red blood cells. The RDW is a key measurement used to assess red blood cell characteristics. An elevated RDW result indicates a wider-than-normal variation in the size and volume of the red blood cells within the sample. This measurement serves as an early indicator of underlying conditions that affect how the body produces or maintains its oxygen-carrying cells.
Understanding Red Cell Distribution Width (RDW)
The RDW is a laboratory measurement that quantifies the degree of disparity in the size of a person’s red blood cells, which are also known as erythrocytes. Normally, these cells are uniform in size. A low or normal RDW value confirms this expected uniformity, meaning most cells are nearly identical in volume.
A high RDW signals a condition called anisocytosis, which is the presence of red blood cells of unequal size. This variation is significant because the primary purpose of red blood cells is to transport oxygen throughout the body. When cell size is highly inconsistent, their ability to function efficiently and circulate smoothly through small capillaries can be compromised. The RDW is calculated by analyzing the distribution curve of cell sizes, where a wider spread indicates greater variation.
Why RDW Levels Become Elevated
An elevated RDW commonly reflects a disturbance in erythropoiesis, the body’s process of producing new red blood cells. Nutritional deficiencies are among the most frequent causes, as they impair the bone marrow’s ability to synthesize healthy, consistently sized cells. Deficiencies in iron, Vitamin B12, or folate each lead to distinct cellular changes that increase size variability.
Iron deficiency prevents the proper formation of hemoglobin, causing the body to produce smaller-than-normal red blood cells (microcytes). When these small cells mix with older, normal-sized cells, the range of sizes widens significantly, increasing the RDW. Conversely, a lack of Vitamin B12 or folate disrupts DNA synthesis, leading to the creation of overly large red blood cells (macrocytes). The coexistence of these large cells with normal cells also creates a mixed population, elevating the RDW.
A high RDW can also result from chronic diseases that interfere with red blood cell turnover and production. Conditions such as chronic liver disease, kidney disease, or persistent inflammatory disorders affect the bone marrow environment. These systemic issues disrupt the steady, uniform process of cell creation and maturation.
External factors can temporarily increase the RDW value. A recent blood transfusion introduces donor cells that may differ slightly in size from the patient’s own cells, leading to a mixed distribution. Medications or the recovery phase following acute blood loss can also cause the bone marrow to release a rapid, heterogeneous mix of cells, resulting in a transiently elevated RDW.
The Role of RDW in Diagnosis and Next Steps
Interpreting a high RDW result requires looking at other Complete Blood Count measurements, particularly the Mean Corpuscular Volume (MCV). The MCV measures the average size of all red blood cells. Combining MCV with RDW (which measures variability) helps healthcare providers narrow down the potential underlying cause. For instance, a high RDW combined with a low MCV strongly suggests iron deficiency anemia.
Alternatively, a high RDW paired with a high MCV points toward a megaloblastic anemia, such as one caused by a Vitamin B12 or folate deficiency. A high RDW can also occur when the MCV is within the normal range, suggesting a mixed deficiency or chronic condition. In this scenario, the average cell size appears normal because small and large cells balance each other out, but the RDW flags the size variation.
Following an elevated RDW, a healthcare provider typically orders specific follow-up tests to pinpoint the exact cause. These tests may include a serum ferritin level to check iron stores or direct measurements of Vitamin B12 and folate concentrations. A peripheral blood smear examination may also be requested to visually confirm varied-sized cells and look for abnormal shapes. Since RDW is not a diagnostic tool on its own, it serves as a valuable signal that guides the next steps in testing.