Red Cell Distribution Width – Understanding Your Blood Test Result

Section Editor: Christopher McCudden Ph.D., DABCC, FADLM, FCACB
September 15, 2026

RDW stands for red cell distribution width. It measures how much your red blood cells vary in size. Most blood tests report an average, but RDW reports variation, which makes it unusual. It appears automatically on a complete blood count, often shortened to CBC, and is never ordered on its own.

This article explains what RDW measures, what a high result can mean, and why it is read with MCV rather than on its own.

The reference range that applies to your result is the one printed on your laboratory report, not the typical ranges shown here. Reference ranges vary between laboratories based on the equipment used, the population tested, and individual factors such as age, sex, and pregnancy status. Always compare your result to the reference range printed on your own report, and discuss any abnormal result with your doctor.

What does the RDW test measure?

Healthy red blood cells are made in the bone marrow and come out at a fairly consistent size. If you could line them up, most would look much the same as one another.

When something disturbs how red blood cells are made, the marrow starts producing cells of differing sizes. Some may be small, some normal, some large, all circulating at the same time. RDW puts a number on how mixed that population is. A low RDW means the cells are uniform. A high RDW means they vary more than usual, a state doctors call anisocytosis.

Your report may show RDW as a percentage, printed next to a reference range. A typical adult range is about 11.5 to 14.5 percent, and exact ranges vary by laboratory. Some reports also provide a second version, labeled RDW-SD, measured in femtolitres rather than as a percentage.

Why is RDW read together with MCV?

RDW on its own says very little. Its value comes from what it adds to MCV, the measurement of average red blood cell size.

MCV gives the average, and RDW says how reliable that average is. If MCV is low and RDW is high, the population contains a mixture of small and normal cells, which fits iron deficiency developing over time. If MCV is low and RDW is normal, the cells are uniformly small, which fits an inherited condition such as thalassemia trait present since birth. Those two possibilities look similar on MCV alone, and RDW helps separate them.

This is also why an average can be misleading on its own. If someone has both iron deficiency, which makes small cells, and vitamin B12 deficiency, which makes large ones, the MCV can sit squarely in the normal range. The RDW will be high because the population is really mixed, and that raised RDW is the clue that something is going on.

Why does RDW often change first?

Red blood cells live for around four months, and the marrow replaces them gradually. When a new problem begins, the cells made from that point on are affected while the older, normal cells are still circulating.

That mixture raises the RDW before it shifts the average enough to affect the MCV. So in early iron deficiency, a raised RDW with a still-normal MCV and a still-normal hemoglobin is a recognized pattern. This is one reason a doctor may investigate a raised RDW even when everything else on the count looks fine.

How is the RDW test performed?

RDW is measured from a small blood sample, usually taken from a vein in your arm. No fasting is needed. The lab calculates it automatically as part of a complete blood count.

If RDW is raised, your doctor may ask the laboratory to examine a blood smear. This is a drop of blood spread on a glass slide and viewed under a microscope. A smear shows the variation directly rather than as a number.

What do RDW results mean?

A normal RDW — a result inside the reference range means your red blood cells are reasonably uniform in size. Together with a normal MCV and hemoglobin, this is reassuring.
A high RDW — a result above the reference range means your red blood cells vary more in size than expected. This is a signal to look at the rest of the count, not a diagnosis in itself. The commonest explanations are nutritional, particularly low iron, vitamin B12, or folate.
A low RDW is not clinically meaningful. Uniform cell size is normal, and laboratories do not usually act on a low result.

A mildly raised RDW with a normal hemoglobin and a normal MCV is a common finding. It is frequently followed with a repeat test rather than an immediate investigation.

What conditions and factors can affect RDW?

Common reasons RDW can be high

Iron deficiency — the most frequent cause, and often the earliest test to change.
Vitamin B12 or folate deficiency — which produces large cells alongside the existing normal ones.
Recent blood loss — as the marrow responds by releasing young cells, which are larger.
Recovery from anemia — when treatment starts working, new healthy cells join the older abnormal ones, and RDW rises temporarily. In this situation a rising RDW is a sign of improvement.
A recent blood transfusion — which mixes donor cells with your own.
Liver disease — which alters the red blood cell membrane.
Conditions in which red blood cells are broken down early — where the marrow releases young cells to keep up.

Because so many of these are common and benign, a raised RDW is best thought of as a prompt to look further rather than as bad news. It is a sensitive signal, which means it changes readily, including for reasons that resolve on their own.

What other tests are ordered with RDW?

MCV — the essential partner, as described above.
Hemoglobin and hematocrit — which show whether anemia is present.
Ferritin and iron studies — the usual next step when iron deficiency is suspected.
Vitamin B12 and folate — when a vitamin cause is being considered.
Reticulocyte count — a measure of how many new red blood cells the marrow is releasing.
A blood smear — to see the variation directly.

What happens after an RDW test?

If the rest of your complete blood count is normal and you feel well, a mildly raised RDW is often repeated in a few months rather than investigated immediately.

If your hemoglobin is low as well, your doctor reads the RDW alongside the MCV to decide what to test next. That usually means iron studies, and sometimes vitamin B12 and folate. If you are already being treated for anemia, a rising RDW early in treatment is expected and usually welcome, because it means new cells are being made. Your doctor will be watching your hemoglobin to confirm that.

Questions to ask your doctor

What was my RDW, and what is the reference range at this laboratory?
What was my MCV, and what do the two results mean together?
Is my hemoglobin normal?
How does this compare with my previous results?
Do I need iron studies, or vitamin B12 and folate testing?
Could this be an early sign of something before other tests change?
Do you want to repeat the test, and if so, when?
If I am being treated for anemia, does this result mean the treatment is working?