An elevated Red Cell Distribution Width (RDW), reported as part of a standard Complete Blood Count (CBC), signals that something is affecting the production or survival of your red blood cells. However, it is not a diagnosis on its own. Understanding what this measurement represents and how it relates to other blood markers is the first step in addressing the finding.
What Red Cell Distribution Width Measures
The RDW test measures the variation in size and volume of your red blood cells (RBCs) in a blood sample. Healthy red blood cells are typically uniform in size. The RDW is reported as a percentage, calculated based on the difference between the smallest and largest red blood cells present.
A high RDW percentage indicates a wider spread in cell sizes, meaning there is a significant presence of both smaller and larger cells than the average. This variation in size is known medically as anisocytosis, which is detected by automated analyzers.
Interpreting an Elevated RDW Result
An elevated RDW flags the presence of anisocytosis, indicating that the red blood cells circulating in your bloodstream are not all the same size. This variation occurs because the bone marrow is producing new cells that are different in size from the older, pre-existing cells. The disparity in size suggests the body is under stress while attempting to generate new red blood cells.
The RDW provides information about the variation in size, which is distinct from the Mean Corpuscular Volume (MCV). The MCV measures the average size of your red blood cells. A normal MCV can mask the presence of two different cell populations, such as both large and small cells. A high RDW, even with a normal MCV, suggests a mixed picture where different sizes of cells are averaging out, providing an important clue that red cell production is experiencing a problem.
Underlying Causes of a High RDW
The most common reasons for an elevated RDW are related to nutritional deficiencies, which impair the bone marrow’s ability to create uniform red blood cells.
Nutritional Deficiencies
Iron deficiency anemia is a frequent cause, where a lack of iron prevents the proper formation of hemoglobin, leading to the creation of smaller-than-normal red cells (microcytes). This pattern often presents as a high RDW alongside a low MCV.
Deficiencies in Vitamin B12 or folate, which are necessary for DNA synthesis, also commonly cause a high RDW. When these nutrients are lacking, the red blood cell precursors in the bone marrow cannot divide properly, resulting in the release of abnormally large cells (macrocytes) into the blood. This condition, known as megaloblastic anemia, typically shows a high RDW combined with a high MCV.
Chronic Conditions
A high RDW can be associated with various chronic conditions that affect the bone marrow or red cell lifespan. Chronic liver disease or kidney disease can impair the body’s ability to regulate red blood cell production, leading to size variation. Inflammation from chronic diseases can also suppress the bone marrow, contributing to ineffective red cell formation and a resultant elevated RDW.
Blood Disorders
Certain blood disorders are also characterized by an elevated RDW due to ineffective or faulty red blood cell creation. Myelodysplastic Syndromes (MDS) are a group of bone marrow disorders where the blood cells do not mature correctly, causing significant size and shape variability. While some inherited conditions, like the thalassemia trait, can cause small red cells (low MCV) with a normal RDW, other more complex or combined anemias will cause the RDW to rise.
Next Steps After Receiving High RDW Results
The presence of an elevated RDW requires a thorough evaluation by a healthcare provider. Your doctor will interpret the RDW in context with your other CBC results, particularly the MCV and your hemoglobin and hematocrit levels. The specific combination of high RDW with low, normal, or high MCV helps narrow down the list of potential underlying causes.
A physician will likely order follow-up blood work to pinpoint the exact issue. These tests often include iron studies, such as serum ferritin and transferrin saturation, and measurements of serum Vitamin B12 and folate levels. A peripheral blood smear may also be ordered to visually confirm the anisocytosis and look for other abnormal cell shapes. Treatment involves addressing the root cause that is stressing red cell production, such as nutritional supplementation or managing an underlying chronic disease.