Red cell size spread RDW-CV
How mixed the sizes of your red cells are.
Also printed asRed Cell Distribution Width · RDW · RDW-SD · Red Cell DW
What it measures
How much your red cells vary in size from one another.
Rises before haemoglobin falls, because new small or large cells mix with the old ones - an early warning of a changing supply.
Why it matters for how long you stay well
It widens as iron runs out, because the newest cells come out smaller than the older ones, so it can flag a shortage months before haemoglobin falls.
How it shows up in daily life
A wide spread often appears before you feel anything, which is what makes it useful.
High and low readings
When it reads high
Mixed cell sizes suggest a shifting iron, B12 or folate picture, or a recent recovery.
When it reads low
Uniform cell size; a low value is normal.
What moves it
Daily life
- iron stores running down
- B12 and folate intake
- recent blood loss
Read it with
Markers that move this one, or that this one moves. One number read alone can mislead.
- Ferritin, your iron storeObserved association
Ferritin pulls it down
As iron runs out, the newest cells come out smaller than the ones already circulating. Mixed sizes widen the spread, often before haemoglobin has moved at all.
- Vitamin B12Observed association
Vitamin B12 pulls it down
New oversized cells mixing with normal ones widens the spread of red cell sizes. So a widening spread can be the first hint of a B12 shortage.
Common questions
What does RDW-CV measure?
How much your red cells vary in size from one another. Rises before haemoglobin falls, because new small or large cells mix with the old ones - an early warning of a changing supply.
What does a high Red cell size spread result usually mean?
Mixed cell sizes suggest a shifting iron, B12 or folate picture, or a recent recovery.
What does a low Red cell size spread result usually mean?
Uniform cell size; a low value is normal.
What is the usual reference range for Red cell size spread?
A broad adult reference band is 11.5–14.5 %. Labs differ, so read your result against the band printed on your own report.
Educational only, not a diagnosis. A single number means little without your history and the rest of your panel, so discuss your own results with your physician.
Typical adult reference band
- Adults
- 11.5–14.5 %
Labs differ. The band printed on your own report is the one that applies to your result.
Reported in %
Ageing relevance
5/5
A direct input to a validated biological-age algorithm.
One of the nine blood inputs to the PhenoAge biological-age clock (Levine et al., Aging, 2018).
Worth knowing
It often widens before the average cell size changes, because new cells differ from the ones already in circulation.
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