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Anemia

RDW

RDW shows how much your red blood cells vary in size from one another.

What is RDW?

RDW measures how much your red blood cells differ in size from one another. A low value means they are all roughly the same size; a high value means small and large cells are mixed together. That spread rises as soon as your bone marrow makes cells under changing conditions. In early iron, B12, or folate deficiency, new cells of a different size appear alongside the existing ones — so RDW can rise before MCV or haemoglobin shift. That makes it an early signal.

Why is RDW relevant?

RDW mainly helps separate different types of anaemia, together with MCV. Small cells (low MCV) with a high RDW classically fit iron deficiency; small cells with a normal RDW point more toward an inherited variant. Large cells (high MCV) with a high RDW fit a B12 or folate deficiency. If you start iron or B12 supplementation, a temporarily rising RDW is actually good news: new, healthy cells of a different size are joining the mix. Once production stabilises, the value normalises again.

RDW high or low — what it means

Always read RDW together with MCV and haemoglobin — on its own it says little. The MCV + RDW combination is the starting point: follow up with ferritin for small cells, or with B12 and folate for large cells. A mildly elevated RDW with an otherwise normal blood count and no symptoms is not uncommon and calls for a repeat, not immediate action. A persistently elevated RDW in someone with anaemia or fitting symptoms does warrant follow-up.

RDW reference ranges

Normal (adults)Common reference range across most Dutch labs (RDW-CV).11.5-14.5 % CV
Adults (UMC Utrecht)Academic lab; slightly narrower range — illustrates how cutoffs vary by lab.10.5-13.5 % CV

RDW is usually reported as RDW-CV in % (coefficient of variation). Cutoffs vary by lab and analyser; some labs also report RDW-SD in fL. Always read the reference value on your own report.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.

Read about our scientific approach →Read the guide: Energy →

Frequently asked questions

What is a normal RDW value?

At most Dutch laboratories a normal RDW (RDW-CV) sits between 11.5% and 14.5%. Some labs use a slightly different range — UMC Utrecht, for example, uses 10.5-13.5% in adults. Within the normal range your red blood cells are fairly uniform in size. Always check the reference value on your own report, as it depends on the lab and the analyser used.

What does a high RDW value mean?

An RDW above roughly 14.5% means your red blood cells vary widely in size — small and large cells are mixed together. This is often an early signal of an emerging problem in blood production, such as early iron, vitamin B12, or folate deficiency, or recovery after blood loss. RDW can rise before MCV or haemoglobin shift.

What does a low RDW value mean?

A low RDW (below the reference range) means your red blood cells are all roughly the same size. On its own this is not concerning and rarely points to disease. RDW is mainly useful when it is elevated; a low value seldom calls for action.

When is an RDW value concerning?

A mildly elevated RDW with an otherwise normal blood count and no symptoms is not uncommon and mainly warrants a repeat measurement, not immediate action. More concerning is a persistently elevated RDW in someone with anaemia or fitting symptoms (fatigue, breathlessness) — that warrants follow-up of iron status, B12, and folate. Always read RDW alongside MCV and haemoglobin.

Related biomarkers

MCV
MCV is the average size of your red blood cells and helps distinguish the type of anemia.
Erythrocytes (RBC Count)
Erythrocytes are your red blood cells, and the RBC count measures them per volume of blood to gauge oxygen transport.
Hemoglobin
Hemoglobin is the oxygen-carrying protein in your red blood cells and the marker that defines anemia.
Ferritin
Ferritin is the protein that stores iron in your cells, making it the most reliable blood test for your iron stores.

RDW is one of the biomarkers in the Optimize blood test. Book a blood draw at any of 238+ partner labs in the Netherlands, or upload your existing results in the app.

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