The Red Cell Distribution Width, or RDW, is a measurement found within a standard Complete Blood Count (CBC) test, providing specific information about your body’s red blood cells. This metric gives physicians a deeper look into the uniformity of these oxygen-carrying cells, which are fundamental to overall health. As a non-invasive and routine part of a full blood panel, the RDW offers valuable clues about potential underlying health concerns. Understanding what this percentage represents helps clarify how your body is producing and maintaining its supply of red blood cells.
What Red Cell Distribution Width Measures
The RDW quantifies the variation in the size and volume of your red blood cells, a phenomenon known in medicine as anisocytosis. While healthy red blood cells are typically uniform, measuring between 6.2 and 8.2 micrometers in diameter, a high RDW indicates that the cells in your bloodstream are highly diverse in size. This measurement is distinct from the Mean Corpuscular Volume (MCV), which only reports the average size of all red blood cells present.
The test results are often generated by analyzing a histogram. When the cells are largely the same size, their values cluster closely together, resulting in a narrow curve and a lower RDW score. Conversely, a wide, spread-out curve on the histogram means there is a significant mix of very large and very small cells, leading to an elevated RDW value. This size variation is often the earliest sign of a problem with red blood cell production in the bone marrow.
The presence of both newly released, often larger, cells and older, potentially shrinking, cells contributes to a high RDW. This diversity signals that the body’s mechanism for generating red blood cells may be struggling to produce a consistently sized population. Therefore, RDW acts as a statistical measure of how much your red blood cell volumes deviate from the average.
Interpreting Your RDW Score
For most adults, the reference range for RDW typically falls between 11.5% and 15.4%. A result within this range indicates that your red blood cells are relatively uniform in size. A score reported as “High” or “Elevated” means red blood cells show greater variation in size than is considered normal.
An elevated RDW suggests a problem with red cell production or survival, prompting a need for further investigation. The bone marrow may be releasing cells prematurely, or the cells may be developing abnormally due to a lack of necessary nutrients. This greater size disparity can compromise the efficiency of oxygen delivery throughout the body.
If your RDW result is reported as “Low,” it is generally not considered to be clinically significant on its own. Physicians primarily focus their attention on results that are elevated, as this is where the diagnostic value of the RDW lies.
Medical Conditions Linked to Elevated RDW
An elevated RDW score is frequently used alongside other CBC metrics, particularly the MCV, to help differentiate between various types of anemia and other conditions. For instance, Iron Deficiency Anemia, the most common type, is characterized by a high RDW and a low MCV, as the iron shortage causes red blood cells to be both varied in size and smaller than average.
In contrast, a high RDW combined with a high MCV often points toward deficiencies in Vitamin B12 or folate. These vitamins are necessary for proper DNA synthesis during red blood cell maturation, and a deficit causes the cells to grow too large before division, leading to macrocytic anemia. The simultaneous presence of both large and small or normal-sized cells from earlier production cycles results in the high RDW value.
Certain genetic blood disorders also affect the RDW, even if the result is normal or only slightly increased. Thalassemia, an inherited condition affecting hemoglobin production, may present with a normal RDW but a very low MCV, which helps distinguish it from Iron Deficiency Anemia. Sickle Cell Disease, which causes abnormal red cell shape, also frequently results in a high RDW due to the extreme variation in cell form and size.
Beyond anemias, an elevated RDW has been correlated with several chronic health issues. Conditions such as chronic liver disease, heart disease, diabetes, and kidney disease can all cause the RDW to rise.