RDW (Red Cell Distribution Width)

A red cell distribution width (RDW) test measures how much the volume and size of your red blood cells (erythrocytes) varies. Red blood cells are made in your bone marrow (the spongy tissue inside your large bones). They contain hemoglobin, an iron-rich protein that carries oxygen from your lungs to every cell in your body.

Your cells need oxygen to grow, reproduce, and make energy for you to function. Normally, your red blood cells are about the same size. Differences in the size of your red blood cells may affect how well they can deliver oxygen through your body.

An RDW test uses a special graph called a histogram. If your red blood cells are about the same size, they will be close together on the histogram, and the RDW will be low. A high RDW means that the size of your red blood cells varies more than it should.

The sizes will be more spread out on the histogram. This may be a sign of a medical condition.

Why this test is done

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

Symptoms of anemia, which may include:

• Shortness of breath
• Weakness or fatigue
• Headache
• Dizziness
• Arrhythmia (a problem with the rate or rhythm of your heartbeat)

Your provider may also order an RDW test for other reasons, for example if you have:

• A family history of thalassemia, sickle cell anemia, or other inherited blood disorder
• A chronic illness such as Crohn's disease, diabetes, or HIV
• A diet low in iron and other minerals
• A long-term infection
• Excessive blood loss from an injury or surgical procedure

If you have anemia, your RDW test may help them determine what's causing your anemia.

How to prepare

No special preparation is necessary.

Sample type

Blood

Other names for this test

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