RDW: what it is, what it tells you and what a high level means

RDW shows how much your red blood cells differ from each other in size. If it is high, cells of different sizes are mixed in your blood: often a lack of iron or vitamin B12 has just started. If it is low, it is not a problem.

Why we test it

In a healthy person, red cells are all roughly the same size. RDW measures how far they stray from that.

Sometimes your body starts making red cells of a different size, for example because iron is short. The new ones mix with the old ones, and RDW rises.

That is why it can pick up a deficiency early, even while MCV, the average size of your red cells, is still normal.

What it tells you, and what it does not

If it is high, your red cells vary in size. Often a deficiency has just started, or two are present together, for example of iron and vitamin B12.

It does not say what the cause is: MCV, ferritin and vitamin B12 help find it.

If it is low, it is not a problem: it just means your red cells are very alike.

That is why it is read together with other tests:

• MCV: The average size: with RDW it shows where to look
• Ferritin: Whether iron is short
• Vitamin B12: If it is short, red cells come out bigger
• Haemoglobin: Whether there is already anaemia

Normal RDW levels

The lab decides what is normal. RDW-CV is usually 11 to 14%. Your own report may print different limits.

Your report may show two entries. RDW-CV, a percentage, shows how much sizes vary compared with the average size. RDW-SD is in femtolitres (fL), the same unit as MCV: it measures directly how wide the range of sizes is.

They are two different ways of measuring how varied the sizes are, with different numbers and limits. Compare each one with the limits printed next to it.

In the Rosa report you also see your optimal range, read together with MCV, ferritin and vitamin B12.

What your result means

Find the row for your result, in %: the table is for RDW-CV. Which values are dangerous? On its own, RDW usually is not: what counts is finding out why it is high.

• Below your report’s limit, Usually below 11: Low. Your red cells are very alike in size. It is not a problem. Nothing needs doing.
• Within your report’s limits, Usually 11 to 14: Normal. Your red cells are similar in size, as expected. In the Rosa report you also see whether you are in your optimal range.
• Above your report’s limit, Usually above 14: High. Your red cells vary in size. Often a deficiency is just starting, or two are present together. Check your MCV, ferritin and vitamin B12. If any of them is out of range, or they do not explain it, talk to your doctor.

If your RDW is high or low

• High RDW: Your red cells vary in size: often a deficiency is just starting. The causes are below.
• Low RDW: Your red cells are very alike in size. It is not a problem.

High RDW: the causes

The first two are the most common.

• An early lack of iron: New red cells come out smaller and mix with the old ones. Ferritin helps tell.
• Too little B12 or folate: Without these B vitamins, new red cells come out bigger. If iron is short as well, MCV can stay normal.
• Blood lost or donated: Your body quickly makes new red cells, which are bigger. After a donation, iron can take months to come back.
• Iron treatment: Once it starts working, old small red cells and new normal ones sit side by side for a few months. RDW rises, then comes back down.
• Other, rarer causes: Liver disease or long-lasting inflammation. If MCV, ferritin and vitamin B12 do not explain it, the cause needs finding with your doctor.

When to worry

Talk to your doctor if:

• It is high and your haemoglobin is low.
• It is high and your ferritin or vitamin B12 is low.
• It is high and MCV, ferritin and vitamin B12 do not explain why.
• You are a man, or a woman after menopause, and iron is short.
• You see black stools or blood in your stools: call 112 or go to the emergency department.
• You are short of breath at rest, have chest pain or feel faint: call 112 or go to the emergency department.

Frequently asked questions

Low RDW: should I worry? Usually not. It just means your red cells are very alike in size, and nothing needs doing.

High RDW with a normal MCV: what does it mean? The average size is normal, but your red cells come in different sizes. It can be a deficiency that is just starting, or two deficiencies cancelling out: one makes small cells, the other large ones. Ferritin and vitamin B12 help tell.

High RDW and low MCV: what does it mean? Often that iron is short: new red cells come out small and different from the old ones. Talk to your doctor: ferritin helps tell whether that is the case, and then the cause needs finding. With a normal RDW, a low MCV points more towards thalassaemia trait.

Is a high RDW dangerous? Not by itself: it is a sign that your red cells are changing, and what matters is the cause, which can often be corrected. In studies of older people, a higher RDW goes with more health risks. It is a pattern across large groups, not a diagnosis.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it twice a year, with the blood count. After iron treatment it can stay high for a few months, which is normal. If it is high, your doctor decides whether and when to repeat it.

Sources

• Bessman and colleagues, improved classification of anaemias by MCV and RDW. American Journal of Clinical Pathology, 1983. PMID 6881096
• Salvagno and colleagues, red cell distribution width, a simple parameter with multiple clinical applications. Critical Reviews in Clinical Laboratory Sciences, 2015. PMID 25535770
• Patel and colleagues, red cell distribution width and mortality in older adults: a meta-analysis. Journals of Gerontology, 2010. PMID 19880817
• Kiss and colleagues, oral iron after blood donation and how long haemoglobin takes to recover. JAMA, 2015. PMID 25668261