The Red Cell Distribution Width (RDW) is a value reported as part of a standard Complete Blood Count (CBC). It assesses the uniformity of red blood cells, which transport oxygen throughout the body. Healthy red cells are typically similar in size, so the RDW measures the variation in their volume and size within the bloodstream. A narrow range of sizes suggests consistent cell production. An elevated RDW indicates a greater mixture of small and large cells, a condition known as anisocytosis, which can signal an underlying issue affecting red blood cell production or lifespan.
How RDW is Calculated and the Standard Range
The RDW is primarily calculated in two ways, though the most common format reported is the RDW-Coefficient of Variation (RDW-CV), expressed as a percentage. The RDW-CV is derived by dividing the standard deviation of red blood cell volume by the Mean Corpuscular Volume (MCV), the average size of the cells. Because this calculation is relative to the average cell size, it can be influenced by a particularly high or low MCV.
A normal RDW-CV range for adults falls between 11.5% and 14.5%. Another version, RDW-Standard Deviation (RDW-SD), is an absolute measurement reported in femtoliters (fL), with a normal range of 39 to 46 fL. These ranges can differ slightly based on the specific laboratory equipment and the reference population used for testing. A result below the normal range, indicating extremely uniform cell size, is not associated with any specific disease state and is rarely a cause for concern.
Interpreting an Elevated RDW Result
A high RDW signifies that the red blood cell population is heterogeneous, containing an abnormal mix of small, large, and normal-sized cells. This elevated value is often one of the earliest indicators of a developing nutritional deficiency, as the bone marrow begins producing abnormally sized cells before the overall red cell count drops significantly. For instance, iron deficiency frequently causes a high RDW because a lack of iron leads to the production of smaller-than-normal red cells that circulate alongside older, normal-sized cells.
Deficiencies in Vitamin B12 or folate also cause an elevated RDW, but they result in the production of abnormally large red cells. The coexistence of these new, large cells with existing normal cells creates the wide size variation measured by the RDW. A sudden increase in RDW can also follow acute events, such as significant blood loss or a recent blood transfusion, as the introduction of donor cells or the body’s rapid response disrupts cell size uniformity. Chronic conditions like cardiovascular disease, liver disease, and certain inflammatory disorders can also lead to an elevated RDW due to their effects on red blood cell production and lifespan.
RDW and Its Diagnostic Partners
The RDW is rarely interpreted in isolation; its utility lies in its relationship with other red blood cell indices, especially the Mean Corpuscular Volume (MCV). The MCV indicates the average size of the red blood cells, while the RDW measures the variation around that average. Comparing these two values helps healthcare providers narrow down the potential cause of a blood abnormality, such as anemia.
A low MCV combined with a high RDW often points toward iron deficiency anemia, where the body produces small, varied cells. Conversely, a high MCV alongside a high RDW is observed in anemias caused by Vitamin B12 or folate deficiency. If a patient has a normal MCV but an elevated RDW, this combination might suggest a very early stage of a nutritional deficiency before the average cell size has changed, or it could indicate a mixed deficiency where small and large cells average out to a normal MCV. This combined analysis helps distinguish between various types of anemia and other underlying conditions.