What Is a Red Cell Distribution Width (RDW) Test?

What Is RDW?

The RDW test identifies the variation in size of your red blood cells.
The RDW test identifies the variation in size of your red blood cells.

A red cell distribution width (RDW) test is one part of a complete blood count (CBC). That’s a standard test doctors use to check your health by looking at the parts that make up your blood. RDW measures how the sizes of your red blood cells vary. If there’s a big difference in their sizes, you could have a problem like anemia (low red blood cell count). Recent studies show results from an RDW may also help predict how other diseases will affect your body. Here is information that can help you understand what the test is and what the results mean to you.

Why Doctors Look at Your RDW Numbers

Though it’s part of a CBC, your doctor may not always review your RDW results. Your doctor is more likely to look at your RDW numbers if you have:

What the Test Results Mean

The word “width” in the RDW test doesn’t mean the size of individual red blood cells. Instead, it refers to the difference in size from your largest to your smallest red blood cell.

A low RDW means your red blood cells are all about the same size. A high RDW means you have both very small and very large red blood cells.

You may also have a “normal” RDW. A normal RDW range is 12.2%–16.1% for women and 11.8%–14.5% for men. But that doesn’t always mean you’re in perfect health. Even though your red blood cells are all about the same size, they could all be smaller or larger than they should be. If your doctor reviews your RDW because of a health concern and the results are normal, the doctor will likely look at the results of other parts of your CBC for more information.

A high or normal RDW can be a sign of:

A high RDW can tip doctors off about a vitamin or nutrient deficiency earlier than other parts of the CBC test can. RDW is also a key test for telling the difference between types of anemia, which helps your doctor find the right treatment.

What Happens When You Take an RDW Blood Test?

You don’t usually need to do anything special to get ready for an RDW test. If your doctor needs specific results from the rest of your blood test, you may need to stop drinking or eating for a certain amount of time beforehand.

You’ll visit a clinic where a health care professional using a small needle will draw blood from a vein in your arm. Your blood will go into a test tube. Once the test tube is full, the clinician will take the needle out of your arm and cover the area with a piece of gauze and a bandage. The whole process should take less than five minutes.

The clinic will send your blood sample to a lab. The lab will look at it under a microscope to get all the information and then send the report to your doctor.

What Are the Next Steps?

The results of an RDW test are only a small piece of the puzzle when it comes to diagnosing a health problem. Doctors use it alongside other parts of the CBC to get a full picture of what’s going on with your body so they can treat the problem correctly.

References
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Cleveland Clinic Journal of Medicine: “Three neglected numbers in the CBC: The RDW, MPV, and NRBC count.”

MedlinePlus: “RDW (Red Cell Distribution Width).”

Mayo Clinic Laboratories: “Test ID: CBC.”

Medscape: “Red Cell Distribution Width (RDW) Test.”
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