A test to determine red cell distribution width shows variation in the size and volume of a person’s red blood cells.

Results of the test, along with those of other blood tests, can help doctors to confirm the presence of anemia, a condition characterized by a lack of red blood cells.

The red cell distribution width (RDW) test can also help to determine the underlying cause of anemia.

Also, a study in 2010 suggested that the test can be a strong predictor of mortality in people over the age of 45.

In this article, we describe what the RDW test is and how to prepare for it. We also explore the outlooks for people with results in various ranges.

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A doctor will generally use a RDW blood test to help confirm anemia.

The RDW test shows the difference in size between the smallest and largest red blood cells in a sample.

While variation is common, average red blood cells are 6–8 micrometers (μm) in diameter.

Red blood cells transport oxygen from the lungs to the rest of the body.

A protein inside the cells, called hemoglobin, binds to the oxygen and carries it.

Problems with hemoglobin can affect the flow of oxygen throughout the body, as can the size, shape, and health of red blood cells. This can result in a range of health problems.

RDW test results may be higher if more cells are larger or smaller than normal. This can suggest the presence of an underlying condition. However, not all conditions affect RDW.

What does it test for?

A RDW test is typically used to evaluate anemia or conditions such as:

Often, the RDW test is part of a complete blood count (CBC), a test that measures all the blood’s components, including white blood cells, platelets, and hemoglobin.

A doctor may order a CBC if a person has:

  • a diet low in iron, vitamin B-12, or other nutrients
  • a family history of blood disorders, including thalassemia or sickle cell anemia
  • chronic illness, such as diabetes, HIV, or Crohn’s disease
  • dizziness, weakness, pale skin, or other symptoms of anemia
  • major blood loss following injury or surgery

If the results of a CBC show low levels of red blood cells or hemoglobin, this usually suggests anemia. Doctors will then try to determine the cause of the condition using the RDW and other tests.

Having a RDW test requires no special preparation. If the blood is being tested for other factors at the same time, a person may need to fast for several hours before the test. A medical professional will let the person know about these and other requirements ahead of time.

Taking blood for the test is quick and simple. The nurse or medical professional will use a small needle to draw the blood from the arm. A person is likely to feel a slight scratching or stinging sensation when the needle breaks the skin.

The blood sample will flow from the needle into a tube. Once the tube is full, the medical professional will remove the needle. They will often instruct the person to hold a piece of gauze against the site to stop any bleeding, before applying a bandage.

The blood sample is then sent to a lab, where a technician will check for red cell size and distribution.

RDW results describe the variation in size and volume of red blood cells in a sample. Results may be normal or high, and the ranges are nuanced.

Normal RDW range

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A RDW blood test is often included in a complete blood count, which measures all blood components.

A result in the normal range for an adult female will be 11.9–15.5 percent.

For an adult male, it will be 11.8–15.6 percent.

A person with a normal result may still have an underlying condition. So, to gain a complete picture of a person’s health, doctors also consider the results of other blood tests.

Doctors often compare RDW results with those of a mean cell volume (MCV) test. An MCV test measures the average volume of a red blood cell. Results may show:

  • Normal RDW and normal MCV. People with normal results may still have anemia caused by a chronic medical condition or blood loss.
  • Normal RDW and low MCV. This combination may indicate anemia caused by a chronic condition or thalassemia.
  • Normal RDW and high MCV. This can indicate a liver condition or alcohol abuse. Or, a person may have this result because they are on antiviral drugs or chemotherapy. If other blood characteristics are also affected, this can suggest aplastic anemia, a rare disorder caused by inadequate blood cell production.

High RDW count

If results are above the normal range, this is known as a high RDW count. It can indicate:

  • Macrocytic anemia. This is characterized by red blood cells that are larger than average. Macrocytic anemia is linked to a deficiency of folate or vitamin B-12.
  • Microcytic anemia. This involves red blood cells that are smaller than average. It is usually caused by a deficiency of iron, but it may also suggest thalassemia.

To confirm a diagnosis, a doctor will compare the results of the RDW test with those of the MCV measurement.

A person may have:

  • High RDW and normal MCV. This suggests a deficiency of iron, B-12, or folate. It may also indicate chronic liver disease.
  • High RDW and low MCV. This suggests iron deficiency or microcytic anemia.
  • High RDW and high MCV. This indicates a lack of B-12 or folate. It can also suggest macrocytic anemia or chronic liver disease.

In 2010, researchers concluded that a high RDW result might be linked to poor outcomes in people with heart failure, or further cardiovascular events in people who have experienced a heart attack.

Anemia can be treated with medications, dietary changes, and supplements. However, untreated anemia can lead to severe complications. In some cases, it can be fatal.

A RDW test is usually carried out as part of a CBC, which provides a more comprehensive picture of a person’s health. Results can indicate the presence of anemia and help to pinpoint the underlying cause.

After determining the cause of anemia, a doctor can develop a plan to treat the condition.