Red cell distribution width (RDW) tests indicate variations in the shape and size of a person’s red blood cells. They can confirm the presence of anemia and can also help determine the type or cause of the condition.

Doctors often include RDW as part of a complete blood count (CBC), which measures the amount of red blood cells, white blood cells, and platelets in the blood.

This article explores the RDW test and how to prepare for it. We also examine the outlook for people with RDW results in various ranges.

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

Red blood cells transport oxygen from the lungs. A protein inside the cells, called hemoglobin, binds to oxygen and carries it throughout the body’s tissues.

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

While variations are common, red blood cells, or erythrocytes, typically have a disk shape, with a diameter of 6.2 to 8.2 micrometers.

RDW test results may be higher if more cells are larger or smaller than average. A high RDW may suggest an individual has an underlying health condition.

RDW tests tell doctors if someone might have anemia, and if so, they can help indicate the type. Often, the RDW test is part of a CBC, a test that measures all the blood’s components, including white blood cells, platelets, and hemoglobin.

A CBC can give doctors an idea of what might be causing the anemia. It can also help with diagnosing other conditions, including:

  • diabetes
  • heart disease
  • liver disease
  • cancer
  • thalassemia, an inherited blood disorder that causes decreased levels of hemoglobin

A doctor may order a CBC if a person experiences:

  • dizziness, weakness, pale skin, or other symptoms of anemia
  • a diet low in iron, vitamin B12, 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
  • major blood loss following injury or surgery

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

Having an RDW test requires no special preparation. If a doctor has ordered other blood tests in addition to RDW, the individual may need to fast for several hours before the test. However, the doctor will inform them about this and any other requirements ahead of time.

Taking blood for the test is quick and simple. A healthcare professional uses a small needle to draw the blood from the individual’s arm. This can cause a small scratching or stinging sensation when the needle enters the skin.

A small amount of blood then flows from the needle into a tube. Once the tube is full, the healthcare professional removes the needle and may ask the individual to hold a piece of gauze against the drawing site to stop any bleeding.

They will then send the blood sample to a laboratory, where a technician checks the size and distribution of the RBCs.

The typical range for RDW is 12–15%. This percentage represents how much red blood cells in a given sample deviate from the average size of the blood cells.

Doctors measure the average size of red blood cells via a mean corpuscular volume (MCV) test, which is part of a CBC. The calculation that doctors use for this is (RDW-SD)/(MCV)×100.

A low RDW percentage means that red blood cells are not very different in size from typical measurements. A high percentage means they differ in size more significantly, which can indicate the body is having difficulty making red blood cells.

However, a person with a typical result may still have an underlying condition. To gain a complete picture of an individual’s health, doctors may also consider the results of other blood tests.

A high RDW result is known as a high RDW count. It can help doctors diagnose and differentiate between different types of anemia. The following types of anemia can cause a high RDW count:

  • iron-deficiency anemia
  • macrocytic anemia, which causes red blood cells to become larger than average
  • microcytic anemia, which causes red blood cells to become smaller than average
  • hemolytic anemias, which are due to the body destroying red blood cells faster than it can make new ones

By contrast, other types of anemia, including thalassemia, do not necessarily cause a high RDW.

Doctors can also get more information by looking at how the RDW compares with other tests in a CBC. For example:

  • a high RDW and typical MCV suggests an iron, B12, or folate deficiency, or possibly chronic liver disease
  • a high RDW and low MCV suggests iron deficiency or microcytic anemia
  • a high RDW and high MCV indicates a lack of B12 or folate, macrocytic anemia, or chronic liver disease

Many conditions have links to a high RDW, including:

  • alcohol use disorder
  • autoimmune diseases, such as psoriasis, lupus, or rheumatoid arthritis
  • pneumonia
  • hereditary spherocytosis, an inherited blood disorder
  • anemia related to myelodysplastic syndrome
  • chronic liver disease
  • kidney disease
  • congestive heart failure
  • valvular disease
  • stroke

However, certain factors can affect the result of an RDW test, making it less accurate. An example of this is blood transfusions — receiving donor blood temporarily changes the composition of someone’s blood, confounding the test results.

Because several factors can cause a high RDW result, lowering the variation in red blood cell size varies according to each case. A doctor can advise how best to do this, depending on the root cause.

For those who are susceptible to a high RDW, it may help by:

  • Eating a balanced diet: A nutritious diet can help prevent deficiencies of iron, folate, and vitamin B12, which contribute to raised RDW levels. However, if an individual has digestive issues affecting how they absorb nutrients from food, they may need to get nutrients in another way, such as through B12 injections.
  • Stopping smoking: People who smoke have elevated RDW. Therefore, quitting smoking may help reduce RDW and provide many other health benefits.
  • Avoiding alcohol: Excessive alcohol can damage RBCs and decrease vitamin B12 and folate absorption.
  • Getting enough sleep: A 2015 study involving over 17,000 participants found that getting 7–8 hours of sleep equated to the lowest RDW levels.
  • Exercising regularly: People with sedentary lifestyles have higher RDW levels. In a 2015 study of over 8,000 individuals, researchers associated an increase in the number of weekly workout sessions with a reduced risk of elevated RDW.

The outlook for someone with a high RDW count varies considerably depending on the cause of this test result. In many cases, it indicates reversible and treatable conditions, such as an iron, folate, or vitamin B12 deficiency. Here, changing the diet or taking supplements may be sufficient to treat the anemia.

In other cases, a high RDW has links to worse health outcomes, particularly in those with more serious illnesses. For example, a 2018 study found that cancer patients with a higher RDW had a greater risk of dying.

Additionally, in 2019, researchers concluded that a high RDW count correlates with a higher mortality risk from any cause. So, it is important to speak with a doctor about the next steps after getting an atypical RDW result.

RDW tests measure the variation in size between blood cells in a given sample. The results of an RDW test can indicate the presence of anemia and some other health conditions. It may assist in pinpointing the underlying cause.

Doctors usually carry out an RDW test as part of a CBC to provide a more comprehensive picture of a person’s health.

Following a balanced diet, sleeping regularly, being physically active, and quitting smoking may help lower elevated RDW levels, but this will depend on its underlying cause. People with any concerns about their RDW test results should speak with a healthcare professional.