Red Cell Distribution Width (RDW) is a measurement included in a standard complete blood count (CBC) test, used to assess overall blood health. RDW specifically measures the variation in the size and volume of red blood cells (RBCs) within the blood sample. RBCs carry oxygen throughout the body and are typically uniform in size to perform this job efficiently. A result that falls outside the normal range suggests a lack of uniformity in the red cell population, indicating the body is producing cells that are either too small, too large, or a mix of both.
How RDW is Measured and Standard Values
The RDW is calculated by automated blood analyzers as part of the complete blood count, offering a quantitative measure of red blood cell size variation, a condition known as anisocytosis. In a healthy individual, red blood cells are relatively consistent in volume, but certain medical conditions disrupt this process, leading to a wider range of sizes and a higher RDW result.
The measurement is derived from the standard deviation of the red blood cell volume, which is then mathematically related to the mean cell volume (MCV). Two main forms of RDW are typically reported: RDW-CV (Coefficient of Variation) and RDW-SD (Standard Deviation). RDW-CV is expressed as a percentage and is the more common measurement.
For most adults, the normal reference range for RDW-CV typically falls between 11.5% and 15.4%. This range can vary slightly depending on the laboratory and the specific equipment used. A result within this range means the red blood cells are generally similar in size, clustering tightly around the average volume.
What Elevated RDW Indicates
An elevated RDW, meaning a result above the standard range, indicates a high degree of anisocytosis, or a greater than normal variation in red blood cell size. This suggests a problem with the bone marrow’s ability to consistently produce red blood cells or an issue with the survival and lifespan of the cells already in circulation.
This increase in size variation is often an early sign of a developing condition, sometimes appearing before other red blood cell indices become abnormal. An elevated RDW can signal nutritional deficiencies, such as a lack of iron, vitamin B12, or folate, all of which are required for healthy red blood cell production.
For example, iron deficiency can cause the bone marrow to release smaller-than-normal cells, while B12 or folate deficiency leads to the release of larger cells. A high RDW is considered a non-specific finding, meaning it flags a potential underlying issue that requires further investigation. Beyond nutritional problems, a higher RDW has been associated with conditions that affect red cell turnover, including chronic liver disease, kidney disease, and certain inflammatory disorders.
Using RDW with MCV for Diagnosis
The RDW is rarely interpreted in isolation; its true diagnostic power emerges when analyzed alongside the Mean Corpuscular Volume (MCV). The MCV measures the average size of all red blood cells, whereas the RDW measures the variation in those sizes. By combining these two metrics, healthcare providers can narrow down the potential causes of anemia or other blood irregularities.
This combined analysis helps differentiate between various blood disorders that might otherwise present with similar symptoms. A pattern of low MCV (small average cell size) combined with a high RDW strongly suggests Iron Deficiency Anemia. In contrast, a low MCV with a normal RDW is often seen in conditions like thalassemia, where the red cells are uniformly small.
Another distinct pattern is a high MCV (large average cell size) paired with a high RDW, which commonly points toward deficiencies in Vitamin B12 or folate. This differentiation is a practical tool for clinicians, as it guides the next steps in testing and helps determine the specific type of deficiency or blood disorder present.