RDW (red cell distribution width)
Blood count value that shows how much the size of your red blood cells varies
Also known as
Definition
RDW (red cell distribution width) describes how much the size and volume of your red blood cells (erythrocytes) differ from one another. Normally they are about the same size. The analyzer plots cell sizes as a histogram: if they sit close together, RDW is low; if they are more spread out, it is high. The value is usually part of the complete blood count and is read together with other blood values, for example when looking for the cause of anemia.
| Parameter | Value |
|---|---|
| Unit | % (RDW-CV); as RDW-SD in fl |
| Reference Range | No single range: RDW is usually reported as a percentage and compared with the reference range of the individual lab, which varies by analyzer and method. The range printed on your report is what counts; your doctor interprets the result. |
Agree on your personal target range with your doctor.
↓ What a low value means
A low RDW means your red blood cells are very similar in size. According to MedlinePlus, this is not a sign of anemia and usually not a cause for concern. A normal RDW does not rule out anemia or another condition either, because the cells can still be larger or smaller than usual overall. Discuss any deviation with your doctor.
↑ What a high value means
A high RDW means the size of your red blood cells varies more than usual. Together with other blood values, this can be related to iron deficiency, different types of anemia, thalassemia or sickle cell anemia. MedlinePlus also lists links with chronic liver disease, heart disease, diabetes, kidney disease and cancer, especially colorectal cancer. A single elevated value is not a diagnosis. Discuss any deviation with your doctor.
When to test
RDW is usually measured as part of a complete blood count, for example at a routine checkup or with signs of anemia such as shortness of breath, weakness, fatigue, headache, dizziness or heart rhythm problems. Other reasons include a family history of thalassemia or sickle cell anemia, chronic illness, a low-iron diet, long-lasting infections or heavy blood loss. No special preparation is needed.
Frequently asked questions
What does a high RDW tell me? +
It shows that your red blood cells differ in size. On its own it does not explain a cause. Your doctor reads it together with other values from the blood count and further lab results.
Is a low RDW a concern? +
According to MedlinePlus, a low RDW is not a sign of anemia and is usually not a cause for concern. It only means the cells are very similar in size.
Does a normal RDW rule out anemia? +
No. Even with a normal RDW, red blood cells can be larger or smaller than usual overall. That is why RDW is never assessed on its own but together with the rest of the blood count.
What is the difference between RDW-CV and RDW-SD? +
Both describe the size variation of red blood cells, but in different ways: RDW-CV as a percentage, RDW-SD as the width of the distribution in femtoliters. Which one appears on your report depends on the lab; the ranges are not interchangeable.
RDW (red cell distribution width)
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.