The Red Cell Distribution Width (RDW) is a component of a routine blood test called a complete blood count (CBC). It provides insight into the characteristics of your red blood cells. Analyzing this parameter helps healthcare providers understand red blood cell health, which is important for oxygen transport throughout the body.
What RDW Measures
RDW, or Red Cell Distribution Width, quantifies the variation in the size of your red blood cells. This variation is medically termed anisocytosis. While healthy red blood cells are uniform in size, certain conditions can cause them to become unequal in volume. The RDW value reflects this variation.
RDW differs from other red blood cell parameters, such as Mean Corpuscular Volume (MCV). MCV reports the average size of your red blood cells, providing a single mean value. In contrast, RDW focuses on the distribution of these sizes, highlighting if there’s a wide range of small, normal, and large cells present in your bloodstream. A higher RDW suggests a greater degree of variability in the volume of red blood cells.
Interpreting RDW Values
A normal RDW range signifies that your red blood cells are consistent in size. While the exact normal range can vary slightly between laboratories, it falls between 12% and 15%. This indicates that the red blood cells are relatively uniform in volume, which is characteristic of healthy blood cell production.
An elevated RDW value suggests a wider variation in red blood cell sizes, meaning there’s a mix of different-sized cells. This increased variability can point towards issues with how red blood cells are being produced or broken down in the body. For instance, the presence of both smaller and larger cells can lead to a high RDW.
Conversely, a low RDW value indicates that your red blood cells are very similar in size, showing minimal variation. This finding is not typically a clinical concern or associated with specific medical conditions. However, a normal RDW does not always rule out an underlying issue, as some conditions can present with a normal RDW despite other red blood cell abnormalities.
RDW’s Role in Diagnosis
Healthcare providers use RDW as part of a diagnostic assessment, not a standalone tool. It is interpreted in conjunction with other red blood cell parameters from a complete blood count, such as Mean Corpuscular Volume (MCV), Mean Corpuscular Hemoglobin (MCH), and hemoglobin levels. This combined analysis helps differentiate between various types of anemia and other blood conditions.
For example, a high RDW combined with a low MCV (indicating smaller red blood cells) suggests iron deficiency anemia. In contrast, a high RDW alongside a high MCV (indicating larger red blood cells) suggests anemias caused by deficiencies in vitamin B12 or folate. Conversely, conditions like thalassemia may present with a low MCV but a normal RDW, as red blood cells are consistently small. An elevated RDW can also be an early indicator of nutritional deficiencies, sometimes appearing before other red blood cell parameters change.