RDW blood value: the earliest sign of iron deficiency in athletes

The RDW blood value describes how much your red blood cells differ in size, usually around 11.5 to 14.5 percent. It measures spread, not an average. And spread changes earlier than an average does.

That is exactly why this column is worth your attention as an athlete.

In early iron deficiency your marrow makes new, smaller cells alongside your existing normal ones. Your RDW rises while your MCV still sits neatly inside its range (PMID 23715364). A normal blood count therefore does not rule out a deficiency.

One thing out of the way first: RDW in your blood has nothing to do with the Dutch vehicle authority of the same initials. Same three letters, completely different world.

What exactly is the RDW blood value?

RDW stands for red cell distribution width, the width of the size distribution of your red blood cells. Your analyser measures thousands of cells and calculates how much their size varies. The bigger the difference between your smallest and largest cells, the higher your RDW.

The technical term for that unevenness is anisocytosis. A high RDW literally means your blood consists of unequal cells.

That sounds vague, but it is measurable and it is early. An average hides a mixed population. Spread does not.

RDW-CV or RDW-SD: which one is on your result?

Labs report RDW in two ways, and they produce completely different numbers. RDW-CV is a percentage, usually around 12 to 15. RDW-SD is an absolute measure in femtolitres, usually around 39 to 46. Compare a 13 with a 43 and you are not comparing two results, you are comparing two units.

This is the most common confusion around this value, and no Dutch page names it.

RDW-CV is divided by your MCV, so a low MCV pushes the CV up without the spread genuinely widening. RDW-SD does not have that problem. If you switch labs, check first which of the two you are holding.

What does a high RDW blood value mean?

A raised RDW means several populations of red blood cells are circulating side by side. Usually that happens because your marrow started producing something slightly different from what it made before: smaller with iron deficiency, larger with a vitamin B12 or folate shortage, or younger after blood loss.

In athletes, iron deficiency tops that list, and in female endurance athletes it does so decisively (PMID 29280410).

That second row is what this whole page is about. A raised RDW with a normal MCV is the moment you want ferritin measured, not the moment you walk away relieved.

Why does RDW rise before your MCV falls?

Because a red blood cell lives about 120 days. When your iron runs down, your marrow makes smaller cells today while the normal cells from three months ago are still circulating. Your average therefore moves slowly, but the spread between old and new moves immediately.

Your MCV is an average of that whole mixture. It takes weeks before enough small cells accumulate to drag that average below the line.

Work on red cell indices in iron deficiency therefore describes RDW as a routine parameter with early signal value (PMID 23715364).

Say you train six times a week, you are tired, your haemoglobin is 8.9 and your MCV 86. All green. But your RDW reads 15.3. That is not noise, that is the start of a story.

Does your training affect your RDW?

Indirectly, and more than you would expect. Endurance athletes lose iron through sweat, through the gut and through red cell breakdown at every footstrike. At the same time training expands your plasma volume, making your haemoglobin look diluted so a genuine deficiency disappears easily against that background (PMID 10688281).

RDW is less sensitive to that dilution, because spread does not change when you dilute everything equally.

Which makes this value more informative in a trained body than the values everybody actually looks at.

When should you discuss a raised RDW with your GP?

When the value stays raised on a calm repeat, especially alongside an abnormal MCV, a low ferritin, or symptoms that do not fit your training load. Persistent fatigue, breathlessness during efforts that used to feel easy, or performance dropping without a reason all belong there.

Thuisarts describes an abnormal blood result as a reason to look further rather than a diagnosis, and the NHG guidelines approach anaemia through the same pattern thinking. RIVM stresses that reference values differ per laboratory in blood testing, which matters extra for RDW because CV and SD exist side by side.

Take your whole result, including earlier measurements. With this value the direction over time is worth more than today's number.

Where do you start?

Find which form is printed on your result, CV or SD, and note it down. If your RDW is high with a normal MCV, have your ferritin measured before you do anything else. Then repeat after eight to twelve weeks, so you see a direction instead of a snapshot.

Our complete blood count test reports your red series including RDW, with an assessment by a BIG-registered doctor.

How the values connect is covered in complete blood count explained and in the complete biomarker guide for athletes. For the value that completes the pattern with RDW, read MCH blood value explained and high MCV explained. If your ferritin does come back low, iron deficiency in athletes is the next step.

References

1. Value of red cell distribution width (RDW) and RBC indices in the detection of iron deficiency anemia. Mymensingh Medical Journal. 2013. PMID 23715364.
2. Sim M, et al. Iron balance and iron supplementation for the female athlete. European Journal of Sport Science. 2018. PMID 29280410.
3. Fallon KE, Fallon SK, Boston T. Sports haematology. Sports Medicine. 2000. PMID 10688281.
4. RIVM. Blood testing and reference values. Accessed 2026.
5. NHG. Dutch College of General Practitioners, anaemia guideline. Accessed 2026.
6. Thuisarts. I have anaemia. Accessed 2026.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. For treatment decisions, discuss your results with your GP.