RDW appears on every full blood count, yet it is one of the least explained numbers on the report. Most UK labs consider roughly 11.5–14.5% normal. Above that simply means your red cells are less uniform in size than expected — a state called anisocytosis. The RDW never diagnoses anything by itself; its job is to point at what to check next.
Should I worry about a high RDW?
Usually not. A mildly raised RDW with an otherwise normal blood count is one of the most common minor abnormalities on blood testing, and in many people it settles or stays stable without any disease appearing. It becomes more meaningful when it is new, rising, or paired with other changes — a falling haemoglobin, an abnormal MCV, or symptoms such as fatigue and breathlessness.
What causes a high RDW with normal haemoglobin?
This combination is genuinely useful, because RDW often moves before haemoglobin falls. Common explanations include:
- Early iron deficiency — the single most common cause. New, smaller cells mix with older normal ones, widening the size spread before anaemia develops.
- B12 or folate deficiency — the mirror image: new cells run larger than the old ones.
- Recent blood loss or recovery — a surge of young red cells (reticulocytes) temporarily widens the spread.
- Liver disease, alcohol and some medicines — all can nudge red cell size around.
Can low iron or B12 cause a high RDW?
Yes — they are the two front-runners, and they can coexist. A mixed iron and B12 deficiency can even produce a normal average cell size (MCV) with a strikingly high RDW, because small and large cells average each other out. That is exactly why a high RDW with a "normal" MCV should not be dismissed without checking ferritin, B12 and folate.
Check your blood count and iron together
A full blood count with an iron profile covers the two most common explanations for a high RDW in one draw.
Book this testWhat should I do next?
- Check ferritin (iron stores), B12 and folate — the three usual suspects.
- Compare with any previous full blood count — a stable RDW over years is reassuring.
- If those are normal and the RDW is persistently high or rising, discuss the trend with a doctor rather than retesting endlessly.
When to see a doctor
See a clinician promptly if a high RDW comes with a low haemoglobin, unexplained tiredness or breathlessness, black or bloody stools, or unintended weight loss. Those combinations need a proper look for the underlying cause — most often a fixable deficiency, but occasionally slow blood loss that matters.
Check your blood count and iron together
Book online with OneMedicine — CQC-registered clinic in Birmingham, or a home visit / local clinic nationwide. Doctor-reviewed results, most within 24 hours.
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Specialist interest: Cardiovascular Medicine · OneMedicine, CQC-registered clinic