What is a red cell distribution width test?

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.

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 group of tests that measures the number and type of cells in your blood, including red cells. The RDW test is commonly used to help diagnose or determine what's causing anemia, a condition in which your body does not make enough red blood cells.

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 (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.

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. Results are usually shown as a percentage and compared to a reference range, which may vary depending on the lab used.

A normal RDW result means your red blood cells are within the reference range. Even if your RDW results are normal, your red blood cells may still be larger or smaller than usual. You may still have anemia or another medical condition that may need treatment.

A high RDW result means your red blood cells are higher than the reference range. This means that your red blood cells vary in size more than what is considered normal. This may be a sign of certain conditions, such as:

• Chronic liver disease
• Heart disease
• Diabetes
• Kidney disease
• Cancer, especially colorectal cancer

A low RDW result means your red blood cells are below the reference range. This means that they are similar in size and close together. This is not a sign of anemia and isn't usually something to worry about.

If you have questions about your results, talk to your provider. They 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. This can help diagnose a variety of conditions, including:

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

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 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 changing your eating plan.