The Red Cell Distribution Width (RDW) is a value included in the Complete Blood Count (CBC), a common blood test. The CBC assesses the overall composition of your blood, including white blood cells, platelets, and red blood cells. An elevated RDW result means the value is outside the typical reference range, suggesting a greater than normal variation in the size of your red blood cells. This article provides context for this finding but is not a substitute for professional medical advice.
What Red Cell Distribution Width Measures
The Red Cell Distribution Width measures the variation in the volume and size of red blood cells (erythrocytes). In a healthy person, these cells are relatively uniform in size, allowing them to efficiently transport oxygen throughout the body. The RDW test statistically quantifies how much the individual sizes of circulating red blood cells deviate from the average size.
This measurement is typically reported as a percentage, known as RDW-CV (Coefficient of Variation). A high RDW value indicates a significant mix of large and small cells in the bloodstream, while a normal RDW shows the cells are mostly uniform. Some labs also report RDW-SD, which is an absolute measurement of the difference between the largest and smallest cells.
Interpreting an Elevated RDW
A high RDW value is a direct indicator of unequal red cell size, medically termed anisocytosis. This variation signals a disruption in the body’s ability to produce, mature, or maintain its red blood cell population. When the bone marrow releases red blood cells of different sizes, it suggests an underlying process is affecting the normal lifespan or development of these cells.
Cell size variability matters because the primary function of red blood cells is to deliver oxygen throughout the body. Cells that are too small or too large may not function optimally, potentially impacting oxygen delivery efficiency. Although a high RDW is a non-specific finding, it acts as a sensitive flag, indicating a problem with red blood cell dynamics that requires further investigation.
Common Causes of High RDW
The most frequent causes of a high RDW are nutritional deficiencies, where a lack of specific vitamins or minerals interferes with normal red blood cell production.
Nutritional Deficiencies
Iron deficiency anemia is a primary cause, resulting in smaller-than-normal red blood cells (microcytes). As the body compensates for the iron shortage, it releases these small, iron-poor cells alongside remaining normal-sized cells, creating a varied population and a high RDW.
Vitamin B12 and folate deficiencies result in the production of larger-than-normal red blood cells (macrocytes). Both B12 and folate are necessary for DNA synthesis; a deficiency impairs cell division, leading to the release of oversized cells from the bone marrow. When these large cells mix with older, normal cells, the size distribution widens, causing the RDW to rise. Mixed-deficiency anemia, such as a combined lack of iron and B12, can also occur, simultaneously producing both small and large cells, leading to a marked RDW elevation.
Hemolytic and Inherited Disorders
Other conditions that increase the RDW include hemolytic anemias, characterized by the rapid destruction of red blood cells. When red blood cells are destroyed prematurely, the bone marrow rapidly replaces them with new, often slightly larger and less mature cells. This rapid turnover and the presence of fragmented cells create a wide spectrum of sizes that elevates the RDW.
Inherited disorders like Sickle Cell Disease, where red blood cells are misshapen, also contribute to a high RDW due to the presence of abnormally formed and sized cells. Thalassemia, another inherited condition, typically presents with uniformly small cells and a normal RDW, but certain forms can still lead to size variation.
Chronic Systemic Diseases
A high RDW is also associated with chronic systemic diseases, including chronic kidney disease, chronic liver disease, and heart disease. These conditions often cause chronic inflammation and oxidative stress that negatively impacts the bone marrow’s ability to produce healthy, uniform red blood cells. The RDW in these cases reflects the body’s overall physiological stress and impaired red cell production or survival.
Next Steps After Abnormal Results
A high RDW result is only one piece of the diagnostic puzzle and is interpreted in conjunction with other red blood cell indices, particularly the Mean Corpuscular Volume (MCV). The MCV measures the average size of the red blood cells. Combining this average with the RDW’s measure of variation helps narrow the potential cause and is a fundamental step in forming a differential diagnosis.
For example, a low MCV (small average cell size) combined with a high RDW strongly suggests iron deficiency. Conversely, a high MCV (large average cell size) alongside a high RDW often points toward a vitamin B12 or folate deficiency. If the MCV is normal but the RDW is high, it may indicate an early stage of a nutritional deficiency, a mixed anemia, or a chronic inflammatory condition. Further testing, such as measuring serum ferritin or B12 and folate serum levels, is necessary to confirm the specific cause. Consulting a healthcare provider is the necessary next step to accurately diagnose the underlying condition and determine the appropriate treatment plan.