Red Cell Distribution Width (RDW)
Disclaimer: Information on this page is for educational purposes. Consult a physician to interpret your test results. Lissa Health helps track biomarker trends but does not replace medical advice.
What This Test Measures
RDW (Red Cell Distribution Width) reflects the degree of variation in red blood cell sizes. A high RDW means erythrocytes differ significantly in size (anisocytosis).
Elevated RDW is an early sign of iron deficiency when MCV is still normal. RDW also rises in mixed anemias (iron + B12) and after blood transfusion.
Normal RDW with low MCV may suggest thalassemia rather than iron deficiency — an important diagnostic distinction.
Normal Ranges
| Group | Reference Range |
|---|---|
| Men | 11.5–14.5% |
| Women | 11.5–14.5% |
| Children | 11.5–15.0% |
Reference ranges may vary by laboratory and assay method.
Causes of High Levels
- Iron deficiency (early stage)
- B12/folate deficiency
- Mixed anemias
- Hemolysis, blood loss
- Post-transfusion
Causes of Low Levels
- RDW is rarely below normal; low values are usually not clinically significant
- Thalassemia trait (RDW often normal)
Test Preparation
- No preparation required
Related Biomarkers
| Biomarker | Relationship |
|---|---|
| Mean Corpuscular Volume (MCV) | Mean red cell size |
| Ferritin | Iron deficiency confirmation |
| Reticulocytes | Young RBCs in regenerative anemia |
FAQ
How often should I take this test?
With every CBC, especially when anemia is present.
What should I do if my result is abnormal?
High RDW + low MCV — check ferritin. Normal RDW + low MCV — rule out thalassemia.
Last updated: June 2026