A high result on the Red Cell Distribution Width (RDW) test indicates that red blood cells, which transport oxygen throughout the body, vary significantly in size and volume. This measurement is an automatic part of a standard Complete Blood Count (CBC). A high RDW means your body is producing a diverse population of red blood cells, often serving as a sensitive indicator of an underlying issue affecting cell production or lifespan. This high number is not a diagnosis, but a valuable clue directing a healthcare provider toward further investigation.
What the RDW Measures and Normal Ranges
The RDW test measures the variation in the size of red blood cells, a condition known technically as anisocytosis. A high RDW means there is a greater spread between the smallest and largest cells in the sample. This variation occurs when the bone marrow, the production site for red blood cells, releases both newly formed cells and older cells, or produces cells that are structurally abnormal.
For most adults, the normal range for RDW is typically between 11.5% and 14.5%, though this can vary slightly depending on the laboratory. A percentage above this range indicates that the red blood cell population is heterogeneous, meaning the cells are not uniform in size. The RDW value quantifies the “width” of the distribution curve of red cell volumes, with a wider curve translating to a higher RDW.
Primary Nutritional Causes of High RDW
Nutritional deficiencies are among the most common and treatable reasons for an elevated RDW, as they interfere directly with the healthy maturation of red blood cells. Insufficient iron, which is required to produce hemoglobin, leads to the bone marrow creating smaller-than-average cells, known as microcytes. When these small, poorly formed cells mix with older, normal-sized cells already in circulation, the variation in size increases, resulting in a high RDW.
A lack of Vitamin B12 or folate also impairs red blood cell production, resulting in cells that are larger than normal, called macrocytes. These deficiencies disrupt the DNA synthesis needed for cell division, causing precursors to grow too large before being released from the bone marrow. The presence of these large, immature cells alongside a normal population also results in a high RDW.
The RDW is often considered a sensitive, early indicator because its value can become elevated before other red blood cell parameters show a change. This occurs because the bone marrow begins to produce a mixed population of cells in response to the deficiency, even before the deficiency is severe enough to lower the total red blood cell count.
High RDW in Chronic Disease and Other Conditions
Beyond nutrient deficits, a high RDW can signal underlying systemic issues, particularly those involving chronic inflammation or organ dysfunction. Long-term inflammatory conditions, such as autoimmune disorders or chronic kidney disease, can disrupt the body’s ability to utilize iron or inhibit bone marrow production. This systemic stress leads to the release of dysfunctional red blood cells into the bloodstream.
Conditions affecting the liver, such as chronic liver disease, can also cause an elevated RDW because the liver plays a significant role in red blood cell metabolism and volume regulation. Impaired liver function can lead to the production of abnormally shaped or sized red cells. Similarly, genetic blood disorders known as hemoglobinopathies, like Thalassemia or Sickle Cell Disease, involve a constant production of defective red blood cells that naturally vary widely in size and shape.
Acute events, such as significant blood loss, can temporarily cause a high RDW as the body attempts to rapidly replace lost cells. This rapid production effort results in a sudden influx of new, often slightly larger, young red blood cells (reticulocytes), which mix with the remaining older cells. This mix of cell ages and sizes creates a heterogeneous population, causing the RDW to rise quickly.
Interpreting the Result with MCV and Next Steps
The RDW is rarely interpreted in isolation and is most meaningful when analyzed alongside the Mean Corpuscular Volume (MCV), which measures the average size of the red blood cells. The combination of these two values helps narrow down the potential cause of the size variation. For instance, a high RDW combined with a low MCV strongly suggests iron deficiency anemia.
Conversely, an elevated RDW paired with a high MCV typically points toward a deficiency in Vitamin B12 or folate. If the RDW is high, but the MCV falls within the normal range, it can suggest an early-stage deficiency, the presence of two different deficiencies (e.g., iron and B12), or certain chronic liver diseases. The RDW acts as a pointer, indicating a problem exists with red blood cell production or lifespan.
If your RDW is high, the next step is a consultation with a healthcare provider to determine the specific underlying cause. This typically involves further diagnostic testing, such as specialized iron studies, including ferritin levels, or serum tests for Vitamin B12 and folate. Identifying the cause is crucial, as treatment depends on the diagnosis, whether it requires nutritional supplementation, management of a chronic disease, or addressing a genetic condition.