RDW (red cell distribution width)
RDW measures how much red blood cells vary in size (the range). High RDW means the cells are uneven — often an early hint of a deficiency.
| Rating | Range | What it means |
|---|---|---|
| Good | below 14.5 % | Red blood cell size is uniform (RDW at optimum). |
| Worth improving | 14.5 – 16 % | Mildly elevated RDW — often an early sign of deficiency (iron, B12). |
| Talk to a doctor | 16 % and above | Elevated RDW — we check your iron stores and B vitamins. |
Why it matters
An early elevation points to a deficiency (iron/B12).
If it’s low
Low/normal RDW means uniform cells — in anemia this points more toward thalassemia or chronic disease than a deficiency.
If it’s high
High RDW means varied cell size — often an early sign of iron, B12 or folate deficiency (the new generation of cells differs from the old).
What to do
Read RDW with MCV and hemoglobin. High RDW warns of a deficiency early — check ferritin, B12, folate.
When to see a doctor
High RDW with anemia (look for a deficiency)
Read together with
MCV · Ferritin · Vitamin B12 · Folate
Part of the guide: Complete blood count (CBC) →
Articles about this marker
- Biological age: what it is and how to slow it downSupplements & longevity
- Complete blood count (CBC): what the basic blood count meansMarker explained
- High RDW: what red cell distribution width means on your blood testComplete blood count (CBC)
- MPV blood test: what high or low mean platelet volume meansComplete blood count (CBC)
This content is for information only and is not a substitute for medical advice. Talk to your doctor about your results.
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