Normal RDW Value
The normal Red Cell Distribution Width (RDW) is ≤14.0%, which corresponds to the 95th percentile of the reference population. 1
Definition and Calculation
RDW is calculated by dividing the standard deviation of red blood cell volume by the mean corpuscular volume (MCV) and multiplying by 100 to express as a percentage: RDW (%) = {SD of red blood cell volume (fL)/MCV (fL)} x 100 1, 2
An RDW >14.0% is considered elevated and indicates increased variation in red blood cell size (anisocytosis). 1
Reference Range Considerations
The normal RDW in children has been reported as 13.2 ± 0.9%, though the ≤14.0% cutoff remains the standard threshold for abnormality 3
In healthy adults, the mean RDW is approximately 13.1%, with a normal reference range typically cited as 11-15% 4, 5
The specific RDW value obtained depends on the instrument used, so interpret according to your laboratory's specific reference range. 1, 2
Clinical Significance of Normal vs. Elevated RDW
A normal RDW (≤14.0%) indicates a homogeneous red cell population with minimal variation in cell size. 2
When combined with MCV, RDW provides critical diagnostic information: a low MCV with RDW ≤14.0% suggests thalassemia minor, while a low MCV with RDW >14.0% indicates iron deficiency anemia 1, 2
Important Caveats
Age-appropriate reference ranges should be used in children, as MCV (and potentially RDW) varies with age—MCV is highest at birth, decreases during the first 6 months, then gradually increases during childhood to adult levels 1, 2
RDW can be elevated in numerous conditions beyond anemia, including cardiovascular disease, chronic inflammatory conditions, and metabolic disorders, though these applications extend beyond traditional hematologic diagnosis 6