RDW blood test: normal range and what a high RDW means

An RDW blood test reports the red cell distribution width: how much your red blood cells vary in size from one another. It comes as part of the complete blood count (CBC). A normal RDW means the cells are much the same size. A high RDW means their sizes are more spread out, which often happens while iron is running short, and it helps a doctor work out what is behind an anaemia.

What it measures

The analyser measures the volume of the red cells in a sample and plots them as a histogram. The Mean Corpuscular Volume (MCV) is the average of that spread; RDW describes how wide it is. RDW-CV is given as a percentage: the standard deviation of the cell volumes divided by the MCV, multiplied by 100. Some reports also give RDW-SD, a direct width in femtolitres (fL), with its own range.

Red cells live about three months, so one sample holds cells made over that whole period. When something changes the cells the marrow is making, such as iron running short, new cells of a different size join the older ones and the spread widens, sometimes while the average is still inside its range.

Normal range

RDW ranges depend on the laboratory and the analyser, so read yours against the range printed on your report. A low RDW is not a sign of anaemia and is not usually a concern.

What a high RDW usually points to

A high RDW is read together with MCV, because the pair narrows things down:

• High RDW with low MCV. Typical of iron deficiency, where new small cells mix with older normal ones.
• Normal RDW with low MCV. More typical of thalassaemia trait, where the cells are small but evenly so. A Gulf expert consensus lists a high RDW in iron deficiency and a normal one in thalassaemia trait as one of the differences between the two.
• High RDW with high MCV. Points towards the causes of large cells, such as low vitamin B12 or folate.
• High RDW with normal MCV. Can mean two populations averaging out, for example small cells from low iron alongside large cells from low B12.

MedlinePlus also lists chronic liver disease, heart disease, diabetes, kidney disease and some cancers among conditions linked with a high RDW. A doctor typically reads the rest of the blood count, checks Ferritin, and Vitamin B12 and Folate where the cells are large, and may look at a blood film.

A large US study of 8,175 adults aged 45 and older linked a higher RDW with higher mortality over the following years, even inside the normal range and after leaving out people short of iron, B12 or folate. That is a pattern across a population; it does not tell anyone what their own RDW means. It is also why PhenoAge, a published estimate of biological age built to predict mortality, picked RDW as one of its nine blood markers. Our article on what biological age is and is not explains how such estimates should be read.

What moves it

• Iron, vitamin B12 and folate. A shortage widens it as new cells of a different size join the old.
• Inherited traits. Thalassaemia traits make cells small but fairly uniform. In a UAE study of citizens screened before marriage, the small-cell group's RDW averaged 12.8%, close to the 11.9% of everyone else.
• Time. Because red cells live about three months, RDW reflects that window rather than the last few days.
• The sample. In one analyser study, RDW-CV drifted beyond accepted limits after about 8 hours at room temperature.

Preparing for the test

RDW needs no special preparation. Follow any fasting instructions for other tests in the same draw; plain water is usually allowed. Tell the person taking the sample about supplements you take, including iron, vitamin B12 or folate.

How Clare's panel handles it

In Clare's panel, RDW is measured by the lab as part of the complete blood count, from the same red cell measurements as MCV. When Clare opens, it will sit beside MCV, MCH and Ferritin in one health account, with your earlier results alongside.

Why it matters in the UAE

In the UAE, RDW helps with a common puzzle: small red cells from iron deficiency, from an inherited thalassaemia trait, or from both. A study of 394 healthy young Emirati adults found an RDW above 14.0% in 21% of the women and 7% of the men, alongside iron deficiency anaemia in 16% of the women and an estimated alpha thalassaemia trait in 32% of the men. A Gulf expert consensus notes that many patients in the region have thalassaemia trait and iron deficiency together, which blurs the pattern, and recommends a ferritin test for anyone with small red cells.

Price

RDW is usually sold within the CBC (complete blood count) rather than on its own. When we read public prices on 11 October 2026, a CBC cost from 35 to 149 at 7 UAE providers. VAT was rarely stated, some were starting or offer prices, and home collection can add a fee.

Related tests

RDW is read with Mean Corpuscular Volume (MCV) above all, and with Ferritin, Vitamin B12 and Folate when it is high. The CBC blood test reports it with the other red cell indices.

Why it means more over time

An RDW that climbs between two blood counts says the marrow has started making cells of a different size, sometimes while MCV and hemoglobin still look normal. An RDW that has been steady for years says the opposite. In a healthy adult it is usually rechecked with the next routine blood count; when iron deficiency is being treated, a Gulf consensus suggests checking hemoglobin and iron stores after a month. Read any change with a doctor, with the results in front of you. Clare is opening with a waitlist at clare.health.