RDW (Red Cell Distribution Width)

What is a red cell distribution width test?

A red cell distribution width (RDW) test is a measurement of the range in the volume and size of your red blood cells (erythrocytes). Red blood cells move oxygen from your lungs to every cell in your body. Your cells need oxygen to grow, reproduce, and stay healthy. If your red blood cells are larger than normal, it could indicate a medical problem.

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 components of your blood, including red cells. The RDW test is commonly used to 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 to diagnose:

  • Other blood disorders such as thalassemia, an inherited disease that can cause severe anemia
  • Medical conditions such as heart disease, diabetes, liver disease, and cancer, especially colorectal cancer.

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:

  • Symptoms of anemia, including weakness, dizziness, pale skin, and cold hands and feet
  • A family history of thalassemia, sickle cell anemia or other inherited blood disorder
  • A chronic illness such as Crohn’s disease, diabetes or HIV/AIDS
  • A diet low in iron and minerals
  • A long-term infection
  • Excessive blood loss from an injury or surgical procedure

What happens during an RDW test?

A health care professional will take a sample of your blood by using a small needle to draw blood from a vein in your arm. The needle is attached to a test tube, which will store your sample. When the tube is full, the needle will be removed from your arm. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

After the needle is removed, you’ll be given a bandage or a piece of gauze to press over the site for a minute or two to help stop the bleeding. You may want to keep the bandage on for a couple of hours.

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

You don’t need any special preparations for a RDW test. If your health care provider has also ordered other blood tests, you may need to fast (not eat or drink) for several hours before the test. Your health care provider will let you know if there are any special instructions to follow.

Are there any risks to the test?

There is very little risk to a blood test. You may have 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 health care 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 needing treatment. That’s why RDW results are usually combined with other blood measurements. This combination of results can provide a more complete picture of the health of your red blood cells and can help diagnose a variety of conditions, including:

  • Iron deficiency
  • Different types of anemia
  • Thalassemia
  • Sickle cell anemia
  • Chronic liver disease
  • Kidney disease
  • Colorectal Cancer

Most likely your doctor will need further tests to confirm a diagnosis.

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 doctor may recommend iron supplements, medications, and/or changes in your diet.

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

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