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Hematology

RDW: What the Spread in Your Red Blood Cells Means

RDW (red cell distribution width) is part of your complete blood count and shows how much your red blood cells vary in size. A raised RDW can be an early clue to anaemia or a shortage of iron, vitamin B12 or folate. Your doctor reviews it together with your hemoglobin and MCV.

What It Measures

RDW stands for red cell distribution width and shows how much your red blood cells vary in size relative to each other. Where MCV reflects the average size of your red blood cells, RDW shows how wide the spread around that average is.

The value is calculated automatically during a complete blood count (CBC) and expressed as a percentage. A low or normal RDW means your red blood cells are fairly uniform in size. A high RDW means smaller and larger cells exist side by side, which is called anisocytosis.

Because RDW is assessed together with values such as hemoglobin, hematocrit and MCV, it gives your doctor extra grip when mapping out how your body is producing blood.

Why It Matters

RDW is especially valuable because it often changes before other values in the blood count become abnormal. As a result, a raised RDW can be one of the earliest signals of developing anaemia.

A high RDW usually means your body is producing red blood cells of varying sizes. This happens with a shortage of iron, vitamin B12 or folate, and with blood loss or increased breakdown of red blood cells.

Combined with MCV, RDW helps distinguish the type of anaemia. A low MCV with a high RDW often fits iron deficiency, while a high MCV with a high RDW can point to a shortage of vitamin B12 or folate. Your doctor uses this distinction to look further in a more targeted way.

When to Test

RDW is measured automatically whenever you have a complete blood count, so you do not need to request it separately. A blood count is often ordered for symptoms such as fatigue, paleness, shortness of breath or dizziness that can fit with anaemia.

The value is also useful if you have an increased risk of a shortage of iron, vitamin B12 or folate, for example with heavy menstruation, a plant-based diet or reduced absorption in the gut.

If an abnormality is found, your doctor can use RDW together with follow-up testing to find the cause and monitor any treatment.

Symptoms

Low Levels

A low RDW is a normal finding and means your red blood cells are roughly the same size. There are no symptoms associated with it on its own and usually no cause for concern.

High Levels

A raised RDW causes no symptoms by itself, but the underlying cause often does. If the high RDW fits with anaemia, you may experience fatigue, paleness, shortness of breath on exertion, headaches or reduced concentration. Symptoms depend on how severe the anaemia is and how quickly it developed.

Lifestyle Tips

A varied diet with enough iron (such as in green vegetables, legumes and meat), vitamin B12 and folate supports the production of healthy, uniform red blood cells.

If you have an increased risk of a deficiency, for example through heavy menstruation or a plant-based diet, discuss with your doctor whether further testing or supplementation makes sense. Do not take iron long term on your own without a confirmed deficiency, as too much iron is not without risk. A persistently raised RDW should always be assessed by a doctor.

Frequently Asked Questions

What does a high RDW mean?

A high RDW means your red blood cells differ more than usual in size. This is often an early sign of an iron, vitamin B12 or folate deficiency, but your doctor will interpret it together with values such as hemoglobin and MCV.

Is a low RDW a problem?

A low or normal RDW usually points to red blood cells that are fairly uniform in size and is generally not a cause for concern on its own.

Why is RDW measured alongside MCV?

RDW and MCV together help distinguish between different types of anaemia. MCV shows the average cell size, while RDW shows how much the sizes vary, which gives your doctor more context.

What is a normal RDW value?

RDW is expressed as a percentage and in adults usually lies roughly between 11.5 and 14.5 percent. The exact reference range can differ per laboratory, so always interpret your value against the range shown on your own result.

What does a high RDW with a normal hemoglobin mean?

A high RDW while your hemoglobin is still normal can be an early clue that something is changing in how you produce blood, even before anaemia is present. Your doctor may use this as a reason to look further, for example at your iron or vitamin levels.

Can a raised RDW return to normal?

If the cause is addressed, for example by correcting a shortage of iron, vitamin B12 or folate, RDW can normalise over time. This happens gradually, because your body needs time to make new, uniform red blood cells.