RDW (Red Cell Distribution Width)
RDW measures variation in the size of red blood cells. A higher RDW means cells are mixed sizes (anisocytosis). RDW becomes elevated early in iron deficiency, often before MCV drops, and is increasingly recognised as a general marker of poor health when chronically elevated.
Normal Range
11.5–14.5%
Reference ranges may vary slightly between laboratories. Always interpret your result in context with your doctor.
Why is RDW measured?
RDW is part of every CBC. It is most useful in classifying anaemia and detecting early iron deficiency. Elevated RDW in non-anaemic patients is associated with mortality in heart failure and other chronic illnesses.
High RDW means…
High RDW indicates a mix of different-sized red cells. The most common cause is iron deficiency (early — even before MCV drops). Other causes include B12/folate deficiency, mixed deficiencies, and chronic disease.
Low RDW means…
Low RDW is rare and not clinically meaningful.
Symptoms associated with abnormal RDW
When RDW is high:
- Symptoms of underlying anaemia
When RDW is low:
- No symptoms
How to improve your RDW
- If RDW is rising and MCV is borderline, suspect early iron deficiency — check ferritin.
- RDW elevation in chronic illness predicts worse outcomes; treat the underlying disease.
Frequently asked questions about RDW
Why is RDW useful before MCV changes?
Iron deficiency starts producing smaller new red cells while older normal-sized cells are still in circulation, creating size variation. RDW captures this mix and rises before MCV does.
Medical Sources
This page is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider about your individual results.