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RDW (Red Cell Distribution Width)

What is a Red Cell Distribution Width Test?

A red cell distribution width (RDW) test measures the differences in the volume and size of your red blood cells (erythrocytes). Red blood cells carry oxygen from your lungs to every cell in your body. Your cells need oxygen to grow, make new cells, and stay healthy.

Normally, your red blood cells are all about the same size. A high RDW means that there's a major difference between the size of your smallest and largest red blood cells. This may be a sign of a medical condition.

Other names: RDW-SD (standard deviation) test, Erythrocyte Distribution Width

What is it used for?

The RDW blood test is often part of a complete blood count (CBC), a test that measures many different parts of your blood, including red cells. The RDW test is commonly used to help diagnose anemia, a condition in which your red blood cells can't carry enough oxygen to the rest of your body.

The RDW test may also be used with other tests to help diagnose other conditions, including thalassemia, an inherited disease that can cause severe anemia.

Why do I need an RDW test?

Your health care provider may have ordered a complete blood count, which includes an RDW test, as part of a routine exam, or if you have:

What happens during an RDW test?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This process generally takes less than five minutes.

Will I need to do anything to prepare for the test?

No special preparation is necessary.

Are there any risks to the test?

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.

What do the results mean?

RDW results help your provider understand how much your red blood cells vary in size and volume. Even if your RDW results are normal, you may still have a medical condition that needs treatment. That's why your provider 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 to help diagnose a variety of conditions, including:

  • Iron deficiency
  • Different types of anemia
  • Thalassemia
  • Sickle cell anemia

A high RDW result can also be a sign of other conditions, such as:

Your provider will most likely need more tests to confirm a diagnosis.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a red cell distribution width test?

If your test results indicate you have a chronic blood disorder, such as anemia, you may be put on a treatment plan to increase the amount of oxygen that your red blood cells can carry. Depending on your specific condition, your provider may recommend iron supplements, medicines, and/or changes in your diet.

Be sure to talk to your provider before taking any supplements or making any changes in your eating plan.


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The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.