Red cell distribution width (RDW) tests: What is it?

RDW blood test

The shape and size of a person’s red blood cells are measured using red cell distribution width (RDW) assays. They can both confirm the presence of anemia and assist in determining its kind and cause.

RDW is frequently included in a complete blood count (CBC), which determines the number of red blood cells, white blood cells, and platelets in the blood.

The RDW test and how to prepare for it are discussed in this article. We also look at the prospects for people with RDW scores in a variety of ranges.

RDW blood test

RDW blood test

The RDW test determines the size and shape difference between a sample’s smallest and largest red blood cells.

Red blood cells transfer oxygen from the lungs to the rest of the body. Hemoglobin, a protein found inside cells, binds to oxygen and transports it throughout the body’s tissues.

Hemoglobin problems can impair the flow of oxygen throughout the body, as well as red blood cell size, shape, and health. This can lead to a variety of health issues.

Red blood cells, or erythrocytes, normally have a disk shape with a diameter of 6.2 to 8.2 micrometers, though variances are common.

If more cells are larger or smaller than usual, the RDW test findings may be greater. A high RDW could indicate that a person has a medical condition.

Uses of RDW tests

RDW testing can assist doctors determine if someone has anemia and, if so, which type. The RDW test is frequently included in a complete blood count (CBC), which evaluates all of the blood’s components, including white blood cells, platelets, and hemoglobin.

A complete blood count (CBC) can help doctors figure out what’s causing the anemia. It can also aid in the diagnosis of other illnesses, such as:

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

If a person has the following symptoms, a doctor may request a CBC:

  • 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

Low amounts of red blood cells or hemoglobin on a complete blood count (CBC) may indicate anemia. Doctors utilize the RDW and other tests to try to figure out what’s causing the problem.

Getting ready for an RDW test

An RDW test does not necessitate any particular preparation. If a doctor has ordered additional blood tests in addition to RDW, the patient may be required to fast for many hours prior to the test. The doctor, on the other hand, will tell them of this, as well as any other prerequisites, ahead of time.

The procedure for drawing blood for the test is quick and simple. A tiny needle is used to take blood from the individual’s arm by a healthcare expert. When the needle enters the skin, it may cause a minor scratching or stinging sensation.

The needle then injects a little volume of blood into a tube. When the tube is full, the healthcare provider removes the needle and may instruct the patient to apply gauze to the drawing site to stop any bleeding.

They’ll then send the blood sample to a lab, where a technician will examine the RBC size and distribution.

Normal RDW range

RDW has a usual range of 12–15 percent. This percentage indicates how far red blood cells in a sample depart from the average blood cell size.

A mean corpuscular volume (MCV) test, which is part of a complete blood count (CBC), is used to determine the average size of red blood cells. Doctors use the formula (RDW-SD)/(MCV)100 to determine this.

A low RDW % indicates that red blood cells are similar in size to standard values. A high proportion indicates that they are significantly different in size, which could suggest that the body is having trouble producing red blood cells.

An individual with a typical result, on the other hand, may have an underlying condition. Doctors may also analyze the findings of other blood tests to get a thorough picture of an individual’s health.

Causes of a high RDW blood test

A high RDW count refers to a high RDW result. It can assist doctors in diagnosing and distinguishing between various kinds of anemia. A high RDW count can be caused by the following forms of anemia:

  • 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 psoriasislupus 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

Certain factors, however, can influence the outcome of an RDW test, making it less accurate. Blood transfusions are one example of this: getting donor blood momentarily alters the makeup of someone’s blood, causing test results to be thrown off.

How to Reduce RDW

Because a high RDW result can be caused by a variety of circumstances, reducing the fluctuation in red blood cell size differs from case to case. Depending on the core cause, a doctor can advise you on the best course of action.

It may benefit people who are sensitive to a high RDW by:

  • Eating a balanced diet: A well-balanced diet can help avoid iron, folate, and vitamin B12 deficiency, all of which lead to elevated RDW levels. However, if a person’s digestive difficulties prevent them from absorbing nutrients from food, they may need to supplement their diet with B12 injections.
  • Stopping smoking: People who smoke have elevated RDW. As a result, quitting smoking may lessen RDW while also providing a slew of 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: RDW levels are higher in people who lead sedentary lifestyles. Researchers found that increasing the number of weekly training sessions was linked to a lower risk of high RDW in a 2015 study of over 8,000 people.


The outlook for someone with a high RDW count is dependent on the source of the test result. It often signals reversible and treatable problems, such as a shortage in iron, folate, or vitamin B12. Changing one’s diet or taking supplements may be enough to treat anemia in this case.

In some circumstances, having a high RDW is linked to poor health outcomes, especially in people who have more chronic conditions. According to a 2018 study, cancer patients with a higher RDW had a higher chance of dying.

Furthermore, researchers determined in 2019 that a high RDW count is associated with an increased risk of death from any cause. As a result, speaking with a doctor about the next measures after receiving an abnormal RDW test is important.


RDW tests are used to determine the size differences of blood cells in a sample. An RDW test can reveal the presence of anemia and a variety of other health issues. It might help you figure out what’s causing the problem.

An RDW test is frequently performed as part of a CBC to provide a more complete picture of a person’s health.

Following a well-balanced diet, getting enough sleep, being physically active, and quitting smoking can all help to reduce increased RDW levels, but it all depends on the underlying cause. People who have concerns about their RDW test results should consult a medical expert.