RDW (Red Cell Distribution Width): What Your Result Means
Result guide · Complete Blood Count
Content last reviewed 2026-09-12 · Educational information — not medical advice
Quick answer
RDW measures SIZE VARIATION among red cells — high RDW means cells are heterogeneous (anisocytosis). It's the earliest CBC flag for iron deficiency (new small cells mixed with old normal cells), and a powerful independent predictor of mortality in various conditions.
What is RDW (Red Cell Distribution Width)?
RDW (typical 11.5–14.5%) quantifies the coefficient of variation of red-cell volumes. UNIFORM sizes suggest stable production; WIDE variation suggests mixed populations — new microcytes entering during deficiency alongside pre-existing normocytes, or fragmented cells in hemolysis.
Typical reference interval: Approximately 11.5–14.5%. Elevated >15% generally warrants explanation.. Laboratories differ — the interval printed on YOUR report always governs.
If your RDW (Red Cell Distribution Width) is high
Common reasons RDW (Red Cell Distribution Width) runs above the reference interval:
- Iron deficiency — new small cells mix with pre-existing normal cells — the FIRST CBC change before anemia develops
- B12/folate deficiency — large cells coexist with normal/mixed sizes
- Recent blood transfusion — donor cells differ from recipient cells
- Hemolytic anemias — fragmented cells and reticulocytes increase variability
- Bone-marrow disorders — myelodysplasia, bone-marrow infiltration
- Mortality risk marker — elevated RDW associates with adverse outcomes in heart failure, CAD, and aging — mechanism unclear but robustly reproduced
If your RDW (Red Cell Distribution Width) is low
Common reasons RDW (Red Cell Distribution Width) runs below the reference interval:
- Not clinically significant — low RDW implies very uniform cells — generally benign
- Thalassemia trait — uniformly small cells may show normal or low RDW (in contrast to iron deficiency's HIGH RDW) — a useful discriminator
Symptoms worth knowing
None directly — RDW is a classifier and prognostic marker, not a symptom-producer. Its value lies in anemia pattern recognition and risk stratification.
What usually happens next
Get your RDW (Red Cell Distribution Width) checked
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Frequently asked questions about RDW (Red Cell Distribution Width) results
RDW 15.5 with normal everything else — why?
Early iron deficiency or B12 depletion before full CBC changes — check ferritin and B12.
Does high RDW predict anything bad?
Population studies show elevated RDW predicts mortality in heart failure and elderly — clinical utility beyond flagging underlying deficiency remains debated.
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