RDW Blood Test: What Red Cell Distribution Width Tells You

What Is RDW?

RDW stands for red cell distribution width. It measures how much your red blood cells vary in size. A CBC calculates it by comparing the width of the red cell size curve to the average cell size. A high RDW means the cells vary a lot in size, a pattern called anisocytosis. A low RDW means they are unusually uniform.

The result is often reported as RDW-CV, the coefficient of variation, though some labs also report RDW-SD. RDW is most useful when it is read next to MCV, the average red cell size, because the pair helps separate different types of anemia.

RDW-SD reports the width in femtoliters, while RDW-CV reports it as a percentage. The two are not interchangeable, so compare your result only to the form your report uses.

Normal RDW Range

A typical normal RDW-CV is about 11.5 to 14.5 percent. Reference ranges vary by laboratory, so use the range on your own report. Some labs list slightly different limits, and children can have their own range.

Because red cell size naturally varies a little, the normal range does not start at zero. A result within the range means the cells are about as uniform as expected. Values are usually stable over time in a healthy person.

RDW result	Category	What it suggests
Below 11.5 percent	Low	Red cells unusually uniform in size
11.5 to 14.5 percent	Normal	Typical variation in red cell size
Above 14.5 percent	High	Red cells vary widely in size

A result just outside the range often means little by itself. The RDW becomes meaningful when it is combined with MCV and hemoglobin.

What a High RDW Means

A high RDW means your red blood cells are not all the same size, which is called anisocytosis. Common causes include:

• Iron deficiency, often before the MCV drops
• Vitamin B12 or folate deficiency
• A mix of two deficiencies, such as iron plus B12
• Hemolytic anemia, in which red cells are destroyed faster than they are made
• Recent blood transfusion
• Liver disease
• Bone marrow conditions such as myelodysplastic syndromes

An early iron deficiency can raise the RDW while the MCV is still normal, which makes RDW a useful early clue. When RDW is high, iron studies, ferritin, vitamin B12, and folate are common next steps.

A high RDW often appears before hemoglobin drops, so it can flag a developing deficiency earlier than the other indices. That is why a clinician sometimes repeats a CBC to watch how the value changes over time.

What a Low RDW Means

A low RDW means your red blood cells are more uniform in size than usual. It is rarely a concern on its own and seldom points to a specific condition. In practice, a low RDW is often just a normal variant, though it can appear in some bone marrow or genetic conditions. Clinicians rarely act on a low RDW unless other CBC values are abnormal.

How RDW Fits Into a CBC

RDW is reported on a CBC alongside the red blood cell indices. Its value is in what it adds to MCV:

RDW with MCV	Pattern that may be considered
High RDW, low MCV	Iron deficiency is a common pattern
Normal RDW, low MCV	Thalassemia is more typical
High RDW, high MCV	Vitamin B12 or folate deficiency is common
High RDW, normal MCV	Early deficiency or a mixed picture

These patterns are clues, not conclusions. A clinician uses them to decide which follow-up tests will be most useful. This is why a high RDW-CV is almost always interpreted together with the rest of the CBC rather than alone.

The RDW also helps when two problems are present at once, such as iron deficiency alongside a B12 deficiency, because the mix produces a wider spread of cell sizes than either condition alone.

When to Test and How to Order a CBC

RDW comes automatically with a CBC, so there is no separate test to order. No fasting is needed, and the test requires only a simple blood draw. Testing may be useful if you have ongoing fatigue, pale skin, shortness of breath, or a family history of anemia or a blood disorder.

You can order a CBC through direct-access lab testing at a CLIA-certified lab, visit a local collection site, and view results through a secure online account. A physician's order is not required, and insurance is not needed. If your RDW is high, adding iron studies, ferritin, vitamin B12, and folate to the same order can save a second visit.

How Much Does a CBC Cost?

A CBC that reports RDW through direct-access lab testing commonly costs about $25 to $60. If iron studies, ferritin, vitamin B12, or folate are added to evaluate a high RDW, the total often falls between about $60 and $150. Prices vary by region and by the markers bundled.

A high or low RDW is a clue, not a diagnosis. A clinician interprets it alongside your MCV, symptoms, and history. If your RDW is well outside the reference range or you have persistent symptoms, talk with a clinician.