RDW (Red Cell Distribution Width)
What is a Red Cell Distribution Width Test?
A red cell distribution width (RDW) test measures the differences in the volume and size of your red blood cells (erythrocytes). Red blood cells carry oxygen from your lungs to every cell in your body. Your cells need oxygen to grow, make new cells, and stay healthy.
Normally, your red blood cells are all about the same size. A high RDW means that there’s a major difference between the size of your smallest and largest red blood cells. 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 test that measures many different parts of your blood, including red cells. The RDW test is commonly used to help diagnose anemia, a condition in which your red blood cells can’t carry enough oxygen to the rest of your body.
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, which includes an RDW test, as part of a routine exam, or if you have:
- Symptoms of anemia, including weakness, dizziness, pale skin, and cold hands and feet
- A family history of thalassemia, sickle cell anemia, or other inherited blood disorder
- A chronic illness such as Crohn’s disease, diabetes, or HIV/AIDS
- A diet low in iron and other minerals
- A long-term infection
- Excessive blood loss from an injury or surgical procedure
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. Even if your RDW results are normal, you may still have a medical condition that needs treatment. That’s why your provider 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 to help diagnose a variety of conditions, including:
- Iron deficiency
- Different types of anemia
- Sickle cell anemia
A high RDW result can also be a sign of other conditions, such as:
Your provider will most likely need more tests to confirm a diagnosis.
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 that 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 making any changes in your eating plan.
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.
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 micrometersTrusted Source.
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:
- heart disease
- liver disease
- 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%Trusted Source. 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 averageTrusted Source
- 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, includingTrusted Source 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 psoriasisTrusted Source, lupusTrusted Source, or rheumatoid arthritisTrusted Source
- hereditary spherocytosis, an inherited blood disorder
- anemia related to myelodysplastic syndrome
- chronic liver disease
- kidney disease
- congestive heart failure
- valvular disease
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 changesTrusted Source 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 RDWTrusted Source. Therefore, quitting smoking may help reduce RDW and provide many other health benefits.
- Avoiding alcohol: Excessive alcohol can damageTrusted Source RBCs and decrease vitamin B12 and folate absorption.
- Getting enough sleep: A 2015 studyTrusted Source 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 levelsTrusted Source. In a 2015 studyTrusted Source of over 8,000 individuals, researchers associated an increase in the number of weekly workout sessions with a reduced risk of elevated RDW.