What is RDW in blood? Normal values, too high and too low

What is RDW? RDW stands for red cell distribution width and measures how much your red blood cells differ from each other in size. The range usually runs from 11.5 to 14.5 percent. The higher the number, the more unequal your cells are in size.

Think of it this way. MCV tells you how large an average cell is. RDW tells you whether they are all roughly that size, or whether small and large ones sit side by side.

That distinction makes RDW more useful than most people realise. In early iron deficiency it is often the first value to move, before your MCV or your haemoglobin show anything.

If one index in this family is worth understanding, it is this one.

What does RDW actually measure?

Your bone marrow makes new red blood cells continuously. Under normal conditions they come out roughly the same size. Disturb production, for instance through a shortage of iron or B12, and cells of a different size appear alongside the older ones. The spread widens, and that is what RDW captures.

Doctors call that inequality anisocytosis. The word sometimes appears on your report.

The value belongs to the complete blood count, together with your MCV, MCH and MCHC.

RDW-CV or RDW-SD, what is the difference?

Labs report RDW in two ways. RDW-CV is a percentage and sets the spread against average cell size. RDW-SD is an absolute measure in femtolitres and captures the width of the distribution itself. They describe the same phenomenon, but they are not interchangeable.

This is the confusion I meet most often. Someone looks up a normal value, finds 11.5 to 14.5, and compares it with a result of 42.

Those are two different units, so two different ranges.

So check which of the two is on your form before comparing anything. If it is a percentage, it is RDW-CV.

One practical detail: RDW-CV can stay normal while RDW-SD is already rising, because the percentage moves with average cell size. At a high MCV the CV can therefore look reassuringly low.

What is a normal RDW value?

For RDW-CV most Dutch labs use a range of about 11.5 to 14.5 percent, with barely any difference between men and women. You can look up your own percentage below.

A mildly raised RDW without symptoms and with a normal haemoglobin is not in itself a reason to worry. It is a clue, not a conclusion.

What does a high RDW mean?

A raised RDW means your red blood cells vary considerably in size. The most common cause is a shortage of iron, vitamin B12 or folate, where new cells come out a different size from the existing ones. Blood loss and recovery after a deficiency also produce a temporarily high RDW.

That combination is what makes RDW useful. In iron deficiency it often rises before your MCV falls.

Picture someone with a haemoglobin just inside the range, an MCV of 88 and an RDW of 15.8. Everything looks normal except that one value.

That is precisely the moment a ferritin test earns its place.

RDW has also been studied extensively as a general marker. One review describes its many clinical applications (PMID 25535770), and in hospital populations a high RDW has been associated with a poorer prognosis across various conditions (PMID 34896818, PMID 41043793).

Read that calmly. Those associations come from groups of people who were already ill, and say little about a healthy adult with an RDW of 15. It is not a personal predictor.

What does a low RDW mean?

A low RDW means your red blood cells are strikingly uniform in size. That is almost never a problem and rarely carries medical meaning. No condition presents as a low RDW, so in practice doctors do nothing with it.

If you see a low number, it is not a reason for further testing.

With this index, attention only goes to the upper end.

Which values do you read beside your RDW?

RDW is always read together with MCV. The two combine into four patterns that shorten the list of causes considerably, which is why labs report them side by side. Your ferritin is the logical third value when the combination points towards iron deficiency.

This table is a reading aid, not a diagnosis. The NHG guidelines and Thuisarts describe how GPs work these patterns up in practice.

Read on about MCV, MCH and haematocrit.

To measure RDW alongside your ferritin, that combination sits in our basic health checkup, or you can build it yourself through your own custom blood test.

My advice: if your RDW is raised and you have never had a ferritin measured, that is the next test to discuss with your GP.

This information is general and does not replace medical advice. Blood results from Vitalcheck are reviewed by a BIG-registered doctor. Always discuss an abnormal result with your own GP.

References

• Red blood cell distribution width: A simple parameter with multiple clinical applications. Crit Rev Clin Lab Sci, 2015. PMID 25535770
• Red Cell Distribution Width as a Predictor of One-Year Prognosis and Mortality. J Stroke Cerebrovasc Dis, 2022. PMID 34896818
• Associations between red blood cell distribution width and mortality. Exp Gerontol, 2025. PMID 41043793
• NHG guidelines and Thuisarts, on anaemia in Dutch primary care