Red Cell Distribution Width
RDW· also: Red Cell Distribution Width, RDW-CV
Clinical Overview
RDW measures the variability in red blood cell size (anisocytosis). A high RDW means red cells vary greatly in size, suggesting mixed nutritional deficiencies, early iron deficiency, or active hemolysis. A normal RDW with low MCV points more toward thalassemia than iron deficiency.
Why This Test Matters
RDW is particularly useful for distinguishing iron deficiency anemia (high RDW, low MCV) from thalassemia trait (normal RDW, low MCV). In iron deficiency, new small cells appear alongside old normal cells, creating size variation. In thalassemia, all cells are uniformly small. RDW is also an emerging prognostic marker — elevated RDW is associated with worse outcomes in heart failure, sepsis, and cancer.
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| 11.5 – 14.5 | % | RDW-CV (coefficient of variation) |
What Causes Abnormal Results?
High RDW Causes
- Iron deficiency anemia (most common cause)
- Vitamin B12 deficiency
- Folate deficiency
- Mixed nutritional deficiency
- Hemolytic anemia (with reticulocytosis)
- Myelodysplastic syndrome
- Recent blood transfusion (mixed cell populations)
Low RDW Causes
- Not clinically significant (low RDW means cells are uniformly sized)
Signs & Symptoms to Watch For
How to Prepare for This Test
No preparation needed. Automatically calculated from CBC.
Factors That Can Affect Results
- Recent blood transfusion (introduces different-sized cells)
- Reticulocytosis (young cells are larger, increasing variation)
- Cold agglutinins (clumps may be miscounted)
Related Topics
Frequently Asked Questions
How does RDW distinguish iron deficiency from thalassemia?
Both conditions cause microcytic anemia (low MCV). In iron deficiency, RDW is typically elevated because there is a mix of new small cells and older normal-sized cells. In thalassemia trait, red cells are uniformly small, so RDW is usually normal. A high RDW with low MCV points to iron deficiency; a normal RDW with low MCV points to thalassemia. Ferritin and hemoglobin electrophoresis confirm the diagnosis.
Can a high RDW predict other diseases besides anemia?
Yes. Elevated RDW has been found to be an independent predictor of mortality in heart failure, sepsis, pulmonary embolism, and several cancers. It appears to reflect systemic inflammation, nutritional deficiency, and oxidative stress — factors that impair normal red cell production. While not used as a primary diagnostic test for these conditions, a high RDW in a CBC should prompt investigation of the underlying cause.