Red Cell Distribution Width (RDW)
RDW measures variation in red-cell size. Elevated RDW signals mixed anaemia and — in non-anaemic patients — an independent cardiovascular and mortality risk marker.
What it measures
RDW is the coefficient of variation of red-cell volumes — how much the cells differ in size from each other. A high RDW means the population is heterogeneous (anisocytosis): smaller and larger cells coexisting. Clinically it is the most useful single CBC index for distinguishing types of anaemia, especially when the MCV is borderline normal but the patient is anaemic. Outside anaemia, large studies in the last decade have shown that elevated RDW independently predicts cardiovascular events, heart-failure outcomes and all-cause mortality — likely a non-specific marker of underlying inflammation, oxidative stress and bone-marrow dysfunction.
What a high value can mean
- Mixed nutritional deficiency — combined iron + B12/folate; small and large cells together.
- Early iron-deficiency anaemia — RDW often rises BEFORE MCV falls; valuable early signal.
- Recent blood loss or transfusion — fresh young cells alongside older ones.
- Hemolytic anaemia — reticulocytes inflate the size variation.
- Chronic disease, heart failure, CKD — non-specific marker of underlying systemic illness.
What a low value can mean
- Generally favourable. Low or normal RDW with a low MCV points strongly to thalassaemia trait rather than iron-deficiency.
When to discuss with a doctor
RDW is part of the diagnostic conversation in any anaemia work-up — not a standalone test to interpret. In a non-anaemic adult, a persistently elevated RDW with no obvious explanation is increasingly seen as a 'check on overall systemic health' marker; Mediora.AI surfaces the trend in the early-warning module. Treatment is driven by the underlying cause, not the RDW itself.