RDW (Red Cell Distribution Width)
Reviewed
Also called: RDW-SD, Erythrocyte Distribution Width
A red cell distribution width (RDW) test measures how much the volume and size of your red blood cells (erythrocytes) varies.
Specimen: Blood (from vein)
Why this test may be ordered
- As part of a routine exam
- If you have symptoms of anemia such as shortness of breath, weakness, fatigue, headache, dizziness, or arrhythmia
- If you have a family history of thalassemia, sickle cell anemia, or other inherited blood disorder
- If you have a chronic illness like Crohn's disease, diabetes, or HIV
- If you have a diet low in iron and other minerals, a long-term infection, or excessive blood loss from injury or surgery
Preparation and collection
No special preparation is necessary.
A health care professional will take a blood sample from a vein in your arm, using a small needle. This process generally takes less than five minutes.
There is very little risk to a blood test. You may experience slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
Understanding results
RDW results help your provider understand how much your red blood cells vary in size and volume. Results are usually shown as a percentage and compared to a reference range, which may vary depending on the lab used.
- Normal RDW: A normal RDW result means your red blood cells are within the reference range. Limitation: Even if your RDW results are normal, your red blood cells may still be larger or smaller than usual. You may still have anemia or another medical condition that may need treatment. Next step: If you have questions about your results, talk to your provider.
What does a high RDW mean?
High RDW: A high RDW result means your red blood cells are higher than the reference range. This means that your red blood cells vary in size more than what is considered normal. This may be a sign of certain conditions, such as: chronic liver disease, heart disease, diabetes, kidney disease, cancer, especially colorectal cancer. Limitation: Results are usually shown as a percentage and compared to a reference range, which may vary depending on the lab used. Next step: They will usually look at your RDW results along with the results of other blood tests. The combined test results can show a more complete picture of your red blood cells.- Low RDW: A low RDW result means your red blood cells are below the reference range. This means that they are similar in size and close together. This is not a sign of anemia and isn't usually something to worry about. Limitation: Results are usually shown as a percentage and compared to a reference range, which may vary depending on the lab used. Next step: If you have questions about your results, talk to your provider.
- There is very little risk to a blood test; slight pain or bruising at the needle site may occur but usually resolves quickly.
- Normal RDW does not rule out anemia or other conditions; further evaluation is needed.
- RDW results are interpreted together with other blood tests, not in isolation.
What to do next
If you have questions about your results, talk to your provider. They will usually look at your RDW results along with the results of other blood tests.
- What is a red cell distribution width test?
- Why do I need an RDW test?
- What do the results mean?
Evidence source
Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.
U.S. National Library of Medicine: RDW (Red Cell Distribution Width)