RDW (Red Cell Distribution Width)

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.

Blood draw from a vein in the arm

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 show how much red blood cell sizes vary. Results are usually given as a percentage and compared to a reference range that varies by lab.

• Normal: Your red blood cells are within the reference range in size variation. Limitation: Even if your RDW is normal, your red blood cells may still be larger or smaller than usual, and you may still have anemia or another condition needing treatment. Next step: Routine
• High: Your red blood cells vary in size more than normal, which may be a sign of conditions such as chronic liver disease, heart disease, diabetes, kidney disease, cancer, or other issues. Limitation: This is not a diagnosis; further testing is needed to identify the cause. Next step: Prompt
• Low: Your red blood cells are more uniform in size than normal, which is usually not a concern. Limitation: This is not usually a sign of anemia and is typically not concerning, but discuss with your provider if you have questions. Next step: Routine

• 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

Discuss your RDW results with your healthcare provider, who will consider them along with other tests to determine if further evaluation or treatment is needed.

• What is a red cell distribution width test?
• Why do I need an RDW test?
• What do the results mean?

Evidence source

U.S. National Library of Medicine: RDW (Red Cell Distribution Width)