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

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, cancer, diabetes and liver disease

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

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.

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

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

1. Lee H, Kong S, Sohn Y, Shim H, Youn H, Lee S, Kim H, Eom H. Elevated Red Blood Cell Distribution Width as a Simple Prognostic Factor in Patients with Symptomatic Multiple Myeloma. Biomed Research International [Internet]. 2014 May 21 [cited 2017 Jan 24]; 2014(Article ID 145619, 8 pages). Available from: https://www.hindawi.com/journals/bmri/2014/145619/cta/
2. Mayo Clinic [Internet].Mayo Foundation for Medical Education and Research; c1998-2017. Macrocytosis: What causes it? 2015 Mar 26 [cited 2017 Jan 24]; [about 3 screens]. Available from: http://www.mayoclinic.org/macrocytosis/expert-answers/faq-20058234
3. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; How Are Thalessemias Diagnosed? [updated 2012 Jul 3; cited 2017 Jan 24]; [about 4 screens]. Available from: https://www.nhlbi.nih.gov/health/health-topics/topics/thalassemia/
4. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; How is Anemia Treated? [updated 2012 May 18; cited 2017 Jan 24]; [about 4 screens]. Available from: https://www.nhlbi.nih.gov/health/health-topics/topics/anemia/treatment
5. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; Types of Blood Tests; [updated 2012 Jan 6; cited 2017 Jan 24]; [about 5 screens]. Available from: http://www.nhlbi.nih.gov/health/health-topics/topics/bdt/types
6. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; What Are Thalessemias; [updated 2012 Jul 3; cited 2017 Jan 24]; [about 4 screens]. Available from: https://www.nhlbi.nih.gov/health/health-topics/topics/thalassemia/
7. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; What Are the Risks of Blood Tests? [updated 2012 Jan 6; cited 2017 Jan 24]; [about 5 screens]. Available from: https://www.nhlbi.nih.gov/health/health-topics/topics/bdt/risks
8. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; What Are the Signs and Symptoms of Anemia? [updated 2012 May 18; cited 2017 Jan 24]; [about 3 screens]. Available from:https://www.nhlbi.nih.gov/health/health-topics/topics/anemia/signs
9. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; What is Anemia? [updated 2012 May 318; cited 2017 Jan 24]; [about 3 screens]. Available from: https://www.nhlbi.nih.gov/health/health-topics/topics/anemia/
10. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; What To Expect with Blood Tests; [updated 2012 Jan 6; cited 2017 Jan 24]; [about 5 screens]. Available from: https://www.nhlbi.nih.gov/health/health-topics/topics/bdt/risks