Normal Range for Red Cell Distribution Width (RDW)
The normal range for RDW is 11.0-14.0%, with values ≤14.0% representing the 95th percentile in reference populations and indicating a homogeneous red blood cell population. 1
Standard Reference Values
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. 1
The Centers for Disease Control and Prevention (CDC) establishes normal RDW as ≤14.0%, which corresponds to the 95th percentile cutoff in healthy reference populations. 1
Research studies in normal populations consistently report mean RDW values of approximately 13.2-13.9% with standard deviations of 0.7-0.9%. 2, 3, 4
Clinical Interpretation of Normal Values
A normal RDW (≤14.0%) indicates uniform size of circulating erythrocytes and absence of significant anisocytosis (variation in red cell size). 1
When RDW is normal and combined with normal MCV, this suggests absence of early iron deficiency or vitamin deficiencies. 1
Values between 11.0-14.0% represent the typical range seen in healthy individuals without hematologic abnormalities. 1, 3
Diagnostic Significance of Normal RDW
In microcytic anemia (low MCV), a normal RDW (≤14.0%) helps differentiate thalassemia minor from iron deficiency anemia, as thalassemia typically presents with low MCV but normal or only mildly elevated RDW, while iron deficiency shows elevated RDW (>14.0%). 1, 3, 4
The combination of low MCV with normal RDW strongly suggests thalassemia minor rather than iron deficiency. 1
Research demonstrates that thalassemia trait patients have mean RDW of approximately 15.4% (mildly elevated), while iron deficiency anemia shows markedly elevated RDW of 20.7%. 4
Important Clinical Caveats
RDW should always be interpreted alongside other hematologic parameters including hemoglobin, MCV, and other red cell indices—never in isolation. 1
Even with normal RDW, if anemia is present, minimum workup should include red cell indices, reticulocyte count, differential blood count, serum ferritin, transferrin saturation, and C-reactive protein (CRP). 1
If clinical suspicion for iron deficiency exists despite normal RDW, additional testing such as serum ferritin and transferrin saturation should be pursued, as early iron deficiency may not yet manifest with elevated RDW. 1
RDW values can vary slightly between different laboratory instruments, so values should be interpreted according to the specific laboratory's reference range. 1
A normal RDW with abnormal MCV still warrants investigation for potential underlying conditions. 1