A Complete Blood Count (CBC) includes several measurements that detail the components of the blood. One specific measurement is the Red Blood Cell Distribution Width (RDW). This value provides information about the characteristics of the oxygen-carrying cells in the bloodstream. If the RDW result is elevated, it indicates a change in the uniformity of the red blood cells circulating in the body. A high RDW can serve as an early indicator of various underlying health issues.
What is Red Blood Cell Distribution Width (RDW)?
The Red Blood Cell Distribution Width (RDW) is a standardized laboratory measurement that quantifies the variation in size and volume among a person’s red blood cells (RBCs). Healthy red blood cells are typically uniform in size, generally falling within a small range. The RDW measures how much the sizes of the cells in the sample deviate from this standard dimension. The result is expressed as a percentage representing the breadth of the cell size distribution.
The RDW is often compared with the Mean Corpuscular Volume (MCV), which measures the average size of all red blood cells. The distinction is important for diagnosis. For instance, the MCV might be normal if a person has a mix of very large and very small cells that average out. However, the RDW would be high, reflecting the significant variation in cell sizes.
Interpreting an Elevated RDW Result
A high RDW value indicates anisocytosis, meaning the red blood cells are unequal in size. This finding suggests the cell population includes a broader mix of sizes than normal, with some cells being smaller and others larger. While reference ranges vary, an RDW above the typical adult range of about 11% to 15% is generally considered elevated. The presence of varied cell sizes suggests an issue with red blood cell production or survival.
This variation occurs when the bone marrow, responsible for manufacturing red blood cells, begins producing new cells that differ in size from the existing cells. The bone marrow may be attempting to compensate for an issue by creating cells that are either too small or too large. When these abnormally sized cells mix with older, normally sized cells, the RDW measurement increases.
Common Causes of High RDW
An elevated RDW is frequently associated with nutritional deficiencies that impair the bone marrow’s ability to create uniform red blood cells.
Iron Deficiency Anemia
Iron Deficiency Anemia is a common cause, where the lack of iron prevents proper hemoglobin synthesis. This leads to the production of small, pale red blood cells (microcytes), which circulate alongside existing normal cells, increasing size variability. While the MCV often decreases in later stages, the RDW typically becomes elevated relatively early in the disease process.
Vitamin B12 and Folate Deficiencies
Deficiencies in Vitamin B12 and folate also commonly lead to a high RDW, but they affect cell size differently. These vitamins are necessary for proper DNA synthesis. Their shortage interferes with the final division of red blood cell precursors, resulting in the release of abnormally large red blood cells (macrocytes) into the bloodstream. When these macrocytes mix with normal-sized cells, the RDW increases significantly.
Chronic Diseases
Various chronic diseases can contribute to an elevated RDW through inflammation or direct organ damage. Conditions such as chronic liver disease and kidney disease can disrupt the production and maturation of red blood cells in the bone marrow. Chronic inflammation, often seen in autoimmune disorders or severe infections, can also interfere with iron metabolism and the bone marrow response.
Genetic Conditions
Genetic conditions, particularly those affecting hemoglobin structure or production, can also present with an increased RDW. For example, some forms of thalassemia, inherited blood disorders, cause the body to produce fewer functional hemoglobin molecules, leading to smaller-than-normal red blood cells. The presence of a high RDW combined with other blood metrics helps differentiate these genetic conditions from nutritional anemias.
Clinical Utility and Follow-Up Testing
Healthcare providers use the RDW primarily to differentiate between various causes of anemia, especially when considered alongside the MCV. This combined analysis creates a powerful diagnostic matrix.
A high RDW coupled with a low MCV suggests an iron deficiency, as the cells are small on average and highly varied in size. Conversely, a high RDW paired with a high MCV points toward a megaloblastic anemia, typically caused by a B12 or folate deficiency.
If the RDW is high but the MCV is normal, it may indicate an early-stage nutritional deficiency or a dimorphic anemia, where populations of both small and large cells average out to a normal MCV. An elevated RDW is rarely a standalone finding and serves as a signal to pursue more specific diagnostic tests.
The next steps after an elevated RDW involve specific blood work to pinpoint the underlying cause. This might include tests to measure iron stores (such as serum ferritin) or direct measurements of Vitamin B12 and folate levels. Depending on the patient’s overall health, a provider may also order specialized diagnostic procedures, such as a peripheral blood smear to visually inspect cell shapes or a hemoglobin electrophoresis to check for genetic disorders.