Red cell distribution widthRDW

Red cell distribution width (RDW) measures how much your red blood cells vary in size. Where MCV gives the average size, RDW captures the spread — how uniform or mixed the population of cells is.

It matters because a wider distribution has turned out to be a surprisingly strong, general marker of health: a higher RDW is associated with a range of conditions and with biological ageing, even when the other counts look normal, which is why it appears in biological-age scoring.

In practice, RDW is read alongside MCV — the combination helps narrow down causes of anaemia — and is followed as part of the wider blood picture rather than acted on alone.