The Red Cell Distribution Width (RDW) measures the variation in size of a person’s red blood cells. This value is routinely included as part of a standard complete blood count (CBC), a common panel of tests used to evaluate overall health. The RDW helps medical professionals assess the uniformity of red blood cells, which transport oxygen throughout the body. A result outside the normal range indicates anisocytosis, which can point toward an underlying medical issue.
What the RDW Test Measures
The RDW test provides an objective, statistical measure of how heterogeneous, or varied, the volume of red blood cells is within a blood sample. This measurement is automatically generated by laboratory equipment during a complete blood count. While the average red blood cell volume is measured by the Mean Corpuscular Volume (MCV), the RDW specifically quantifies the spread of the cell sizes around that average.
The technical term for a significant variation in red blood cell size is anisocytosis, and a high RDW value directly correlates with an increased degree of this variation. The RDW is calculated either as a coefficient of variation (RDW-CV) or as a standard deviation (RDW-SD) from the cell volume distribution curve. The RDW-CV is the measurement most commonly reported.
Understanding the difference between RDW and MCV is important for interpretation. The MCV indicates the average volume of the red cells (microcytic or macrocytic). In contrast, the RDW shows the range of sizes. A person could have a normal average size (MCV) but still have a high RDW if they have a mix of very small and very large cells. This ability to detect size variation early makes the RDW a sensitive indicator of certain conditions.
Interpreting RDW Values
RDW results are typically reported as a percentage, and the normal reference range for adults generally falls between 11.5% and 15.4%. This range can vary slightly depending on the specific laboratory and equipment used. When the RDW value is within this range, it indicates that the red blood cells in the sample are mostly uniform in size.
An elevated RDW, meaning a value above the reference range, is the clinically significant finding and signals the presence of anisocytosis. This high value confirms a wide distribution of sizes among the red blood cells, with some being much smaller or larger than normal. A high RDW suggests the body is producing red cells distinctly different in size from the existing, older red cells.
A result that is lower than the reference range is uncommon and is generally not considered clinically significant on its own. It is important to note that RDW is rarely interpreted in isolation; it must be considered alongside the MCV and other components of the complete blood count. For example, a person with a normal RDW may still have a medical condition if their MCV is outside the normal limits, such as in certain forms of thalassemia.
Clinical Conditions Associated with Abnormal RDW
The RDW helps diagnose and differentiate causes of anemia and various nutritional deficiencies. By combining the RDW result with the MCV result, healthcare providers can narrow down the potential underlying conditions, as different diseases produce unique patterns of cell size and size variation. The RDW’s ability to increase early in the disease process, sometimes before the MCV changes, makes it a valuable screening tool.
High RDW and Low MCV
This pattern is most commonly associated with iron deficiency anemia. In this state, the body is unable to produce sufficient hemoglobin, leading to the creation of many small red blood cells (low MCV) alongside older, normal-sized cells, which results in a high variation (high RDW). This combination also helps distinguish iron deficiency from other causes of small red cells, such as thalassemia minor, which typically presents with a low MCV but a normal RDW because all the cells are uniformly small.
High RDW and High MCV
This combination frequently suggests a megaloblastic anemia, which is usually caused by a deficiency in Vitamin B12 or folate. A lack of these vitamins impairs DNA synthesis, leading to the production of fewer, abnormally large red blood cells (high MCV) that vary greatly in size (high RDW). This pattern is a hallmark of these specific nutritional deficiencies.
High RDW and Normal MCV
This result can occur in several situations, including the very early stages of iron, B12, or folate deficiency. At this initial stage, the red cell volume variation (RDW) is already changing, but the average cell size (MCV) has not yet moved outside the normal range. This pattern can also be seen in mixed deficiency states, where a person is simultaneously deficient in both iron and B12, causing the effects on cell size to cancel each other out in the MCV but still result in high size variation.
The RDW may also be elevated in conditions not directly related to nutritional deficiency or anemia, such as chronic liver disease, kidney disease, and chronic inflammatory disorders. This elevation may reflect generalized inflammation, oxidative stress, or dysfunction in the bone marrow’s production of blood cells. An elevated RDW in these contexts can be a significant prognostic marker, reflecting underlying systemic health issues.