The Red Cell Distribution Width (RDW) is a measurement included in a standard Complete Blood Count (CBC) that provides information about the size and variation of red blood cells. While discussions often focus on high RDW values, a low RDW result can be puzzling. This article explains what a low RDW indicates, why it occurs, and how it is typically interpreted in a clinical setting.
Understanding Red Cell Distribution Width Measurement
The RDW test measures the degree of variation in the volume or size of red blood cells (RBCs) in a blood sample. Red blood cells are normally uniform in size, typically ranging from 6.2 to 8.2 micrometers in diameter. The RDW quantifies how tightly clustered these sizes are and is reported as a percentage. The normal reference range for RDW-CV (Coefficient of Variation) is typically between 11.5% and 14.5%, though this varies by laboratory.
A high RDW value indicates a significant size disparity among the red blood cells, a condition known as anisocytosis. This variation often suggests the production of different-sized red blood cells, which can signal an underlying issue like a nutritional deficiency. A normal RDW means the red blood cells are relatively uniform in size. The RDW is an important parameter used alongside other indices to help differentiate between various causes of anemia.
Interpreting a Low RDW Result
A low RDW result is less common than a high result and indicates that the red blood cells in the sample are highly uniform in size. This suggests a homogeneous cell population where the cells are very close to the same volume and diameter.
Results below the established normal range are considered a low RDW. While a high RDW strongly indicates various anemias, a low RDW is often less clinically significant. A low value is not typically associated with any specific disease and is frequently viewed as a desirable or non-pathological finding, especially if all other blood indices are within normal limits.
Specific Conditions Associated with Uniform Red Cells
Although a low RDW is often considered benign, a uniform red cell population is observed in specific hematological conditions, sometimes alongside a low Mean Corpuscular Volume (MCV). When the RDW is low or normal, but the MCV is low (microcytic), it suggests a condition where the red blood cells are uniformly small.
The most notable condition is Thalassemia minor, an inherited blood disorder. Patients produce red blood cells that are smaller than average but remarkably consistent in size. This results in a low MCV and a normal or low RDW, which distinguishes it from Iron Deficiency Anemia, which typically presents with a high RDW.
Other hemoglobinopathies, which are genetic disorders affecting hemoglobin production, may also present with a low RDW. Some types of Sideroblastic Anemia, a disorder where the body fails to incorporate iron into hemoglobin, may also show a homogeneous red cell population. In healthy individuals, an extremely low RDW might simply reflect naturally precise red cell production by the bone marrow, requiring no further investigation.
Next Steps After Receiving a Low Result
The RDW is a single piece of data within the comprehensive Complete Blood Count panel. Its clinical significance must be determined by reviewing other parameters, especially the Mean Corpuscular Volume (MCV), Hemoglobin, and Hematocrit levels. If all other blood counts are normal, a low RDW is generally not a cause for concern and indicates a healthy blood picture.
If the low RDW is accompanied by a low MCV and the patient has no symptoms, a healthcare provider may consider a Thalassemia trait. Follow-up testing, such as Hemoglobin electrophoresis or genetic testing, may be recommended to confirm the diagnosis. Patients must consult with their healthcare provider to have results interpreted in the context of their full medical history, as self-diagnosis based on a single blood index is not appropriate.