What Is Red Cell Distribution Width on a Complete Blood Count?

What to expect when undergoing this test

Red cell distribution width (RDW) is one of the numbers or blood cell indices that is included as part of a complete blood count (CBC), and describes the variation in the size of red blood cells in a sample of blood. A higher RDW means that there is a greater variation in the size of red blood cells than expected. The RDW can be very helpful in distinguishing between the different types of anemia, especially if there is more than one type of anemia present.

Even when blood counts such as the red blood cell count are normal, however, the RDW can be a valuable test. For example, it can predict iron deficiency in pregnant women even before anemia occurs (iron deficiency increases the risk for both mothers and babies). It may also be helpful in estimating heart disease or cancer risk, and some healthcare providers believe, may be a test that assesses overall well-being.

There are limitations in evaluating the RDW such as after a person has had a blood transfusion. RDW may also be referred to as erythrocyte distribution width or RDW-SD (standard deviation test).

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The red cell distribution width (RDW) is done as part of a CBC and is, therefore, a commonly performed test that is used both for screening healthy individuals and to evaluate a wide range of medical conditions.

There are times when healthcare providers may specifically look at the value of RDW:

  • with symptoms of anemia, such as lightheadedness or fatigue
  • to help diagnose the causes of anemia (a wide variation in the size of cells or a high RDW may occur when more than one type of anemia is present)
  • to screen people who have a history of a red blood cell condition such as thalassemia
  • with heart disease (an elevated RDW is a strong predictor for eventual heart failure)
  • to screen for early iron deficiency in pregnant women before anemia occurs
  • to screen for early vitamin B12 and folate deficiency before other signs are noted in the blood
  • to have an idea when further blood tests (peripheral smear) are needed
  • as an adjunct in estimating disease risk (heart disease, cancer, and more) or defining prognosis


RDW may be reported as either standard deviation (SD) or coefficient of variation (CV), but RDW-CV is most common. One standard deviation of RBC volume divided by the MCV times 100.

  • SD / MCV x 100


The RDW is used to describe the amount of variation in the size of red blood cells, with the term anisocytosis used to describe this variation. In other words, talking about significant anisocytosis on a blood smear would mean that the red blood cells vary significantly in size.

Red blood cells are usually fairly uniform in size, and an increase in variation or anisocytosis (an increased RDW) can mean several things. A high RDW can be a sign of some types of anemia as well as a general sign of inflammation in the body.


If an RDW is drawn after a blood transfusion, it won't accurately reflect the RDW of a person's cells. If a lab uses EDTA anticoagulated blood instead of citrated blood, the reading will be falsely high. Since the RDW-CV is calculated using MCV, an error in MCV will result in an error in the RDW.

Similar Tests

Variation in red blood cell size may also be noted visually by looking at a peripheral smear for morphology, though this test is usually done after a CBC to investigate an abnormality.

Complementary Tests

Since the RDW is done as part of a CBC, the number is reported along with several other values and the combination of results is usually used rather than the RDW alone. These include the number of each type of the blood cells and other red blood cell indices.

  • Red blood cells (RBCs)
  • White blood cells (WBCs)
  • Platelets
  • Hemoglobin and hematocrit
  • Mean corpuscular volume (MCV) or the measure of the size of red blood cells
  • Mean corpuscular hemoglobin concentration (MCHC) or the measure of the concentration of hemoglobin in a specific volume of red blood cells
  • Mean corpuscular hemoglobin (MCH), which parallels the MCV and has little value in general
  • Mean platelet volume (MPV), which is the average volume of platelets that can provide clues about many diseases

Additional Tests

In addition to the CBC, other tests that may be ordered to evaluate anemia include a reticulocyte count, blood smear for morphology, iron studies, and more.

Risks and Contraindications

Since the RDW is part of a simple blood test, there are very few risks. Uncommonly people may experience bleeding at the puncture site, bruising (a hematoma), or infection.

Before the Test

There are no special dietary or activity restrictions prior to doing an RDW (CBC). You will need to have your insurance card and it's helpful to provide your healthcare provider with any previous CBC results for comparison.

During the Test

A CBC may be drawn in the hospital as well as many clinics. Before drawing your blood, a lab technician will clean the area (usually an arm) with antiseptic and apply a tourniquet to make the vein easier to see. She will then insert the needle through your skin and into the vein. While the needle is inserted you may feel a sharp (but short) sting, and some people may experience lightheadedness or feel faint.

After the sample is removed, the needle is removed and pressure applied to the puncture wound. A dressing is then applied to keep the area clean and reduce any bleeding.

After the Test

As soon as your blood is drawn, you will be able to return home. Potential side effects may include:

  • Bleeding. Sometimes the area where your blood was drawn will continue to bleed, though this is most common for those who are on blood thinners or have a bleeding disorder. Most often this can be resolved with applying pressure, but if bleeding persists you should contact your healthcare provider.
  • Hematoma. Uncommonly, a large bruise may develop where your blood was drawn. This, again, is more common for those who are taking blood thinners such as anti-platelet medications.
  • Infection. There is a very small risk that an infection could develop as a result of bacteria on the skin being introduced into the body during the blood draw.

Interpreting Results

If your clinic has a lab associated with it, most often you will receive your results shortly after they are completed. In some cases, the blood sample will be sent out to a lab and your healthcare provider will call with results when they are available.

When you receive your results, it's helpful to ask for the exact numbers, including that of your RDW. As discussed below, the RDW may give important information even of the remainder of tests on your CBC are normal.

Reference Range

Reference ranges for RDW can vary somewhat by the laboratory doing the test. Normal red blood cells average between 6 and 8 micrometers in diameter. RDW estimates the variation in sizes of the cells and is given as a percentage. The normal range for RDW is roughly 11.8 to 15.6 percent, and the number often increases with age.

Normal RDW With Anemia

Examples of anemias in which RDW is most often normal include:

  • Thalassemia (some types)
  • Anemia of chronic disease
  • Liver disease
  • Alcohol abuse anemia
  • Aplastic anemia

High RDW

A few types of anemia associated with an elevated RDW include:

  • Iron deficiency anemia including early deficiency
  • Vitamin B12 and folate deficiency
  • Mixed anemias
  • Sickle cell disease
  • Myelofibrosis
  • Cold agglutinin disease

Using RDW and MCV Together

Using the combination of RDW and MCV is very helpful in differentiating some types of anemia which would otherwise be difficult to tell apart. For example, both iron deficiency anemia and thalassemia are usually associated with a low MCV (microcytic anemias), but the two conditions are treated differently. Checking the RDW can help distinguish between these.

Similarly, megaloblastic anemias (such as vitamin B12 deficiency and folate deficiency) and non-megaloblastic anemias (such as anemia related to liver disease) are both associated with a high MCV (macrocytic anemias), but again are treated differently. In this case, the megaloblastic anemias usually have a high RDW and non-megaloblastic a low RDW, helping to make the distinction.

RDW can also be very helpful in mixed anemias. For example, a combination of iron deficiency (microcytic anemia) and folate deficiency anemia (macrocytosis) may have a normal MCV (normocytic anemia), but the RDW will be very high.

The following are examples of what conditions that RDW and MCV rates may indicate. It's important to note that there are exceptions to these general rules—for example, sometimes anemia of chronic disease is associated with a low MCV, and sometimes iron deficiency anemia will show a normal MCV.

  • High RDW and Low MCV: Iron deficiency anemia, sickle cell, beta-thalassemia, or hemoglobin H
  • High RDW and Normal MCV: Early iron deficiency anemia, early B12/folate deficiency, blood loss (chronic), or hemolysis
  • High RDW and High MCV: Vitamin B12 deficiency, folate deficiency, immune hemolytic anemia, or this is a prevalent combination in newborns
  • Normal RDW and Low MCV: Anemia of chronic disease, thalassemia, hemoglobin E trait
  • Normal RDW and Normal MCV: Blood loss (acute), anemia of kidney disease, some abnormal hemoglobins, or spherocytosis
  • Normal RDW and High MCV: Aplastic anemia, liver disease, alcohol abuse, some medications result in this combination (such as chemotherapy or antivirals)

Other Tests

In addition to a CBC, other tests that may be done to help identify anemia include:

  • Reticulocyte Count: The reticulocyte count helps separate anemias into those based on lack of production of red blood cells (normal reticulocyte count) and those in which there is a loss or break down of red blood cells (blood loss or hemolysis) that are usually associated with a high reticulocyte count..
  • Blood Smear: In a peripheral blood smear, the blood sample is viewed under the microscope. In addition to being able to visualize differences in size and shape, other findings may include target cells, nucleated red blood cells, fragmented red blood cells (with hemolysis), and more.
  • Iron Studies: Serum iron and iron-binding capacity and/or serum ferritin can measure iron stores in the body.
  • Vitamin B12: If vitamin B12 deficiency is suspected, a vitamin B12 level will be drawn.
  • Hemoglobin Electrophoresis: This study can find some (but not all) types of thalassemia.
  • Bone Marrow Study: A bone marrow aspiration and/or biopsy may be done to look at the types of cells in the bone marrow and iron stores.

Non-Anemia Uses for RDW

The RDW can be a very helpful number even if there is no evidence of anemia (if the red blood cell count and hemoglobin levels are normal).

The RDW can predict the overall risk of mortality in people over the age of 45 (people with a high RDW are more likely to die earlier on than those who have a lower RDW).

Many studies have been done in the last several years looking at the predictive value of RDW in a wide range of diseases. Some of these include:

  • Heart Disease: RDW appears to be a strong predictor of heart failure in people with heart disease, and also predicts the risk of heart disease developing in people with high blood pressure. A 2014 study found that people with a very high RDW (in the top 5 percent) were 71 percent more likely to have a heart attack than those who had a lower RDW. A high RDW may also help predict the risk of heart disease in people who are infected with HIV,
  • Cancer: Studies have looked at the role of RDW in cancer in a few different ways. With several types of cancer (such as blood-related cancers, lung cancer, and colon cancer), a high RDW may signify a poorer prognosis.

From another angle, researchers have looked at the potential for RDW to predict the risk of cancer in people who do not currently have the disease. For example, they found a dose-dependent relationship between high RDW values in men and postmenopausal women and future cancer risk.

For people who are undergoing evaluation for unintentional weight loss, a high RDW increased the chance that the weight loss was due to cancer.

  • Surgery: Studies looking at different types of surgery have found that RDW may predict the risk of complications after surgery, to the point where it was pointed out that RDW is important for orthopedic surgeons.
  • Sleep: A high RDW is linked to some sleep disorders, such as sleep apnea, and is also elevated in those who get too little or too much sleep or do shift work.
  • Diabetes: People who have elevated RDW appear to have a greater risk of developing diabetes.

This area of research (looking at the role of RDW in evaluating conditions other than blood conditions) is quite new, and it's expected that more information will be available to better understand the potential benefits of looking at RDW in the future.

  • Inflammatory/Autoimmune Conditions: An increased RDW has been associated with a number of inflammatory and autoimmune conditions, ranging from lupus to autoimmune thyroiditis.


Follow-up testing if the RDW is abnormal will depend on many factors. Be sure to discuss your results with your healthcare provider and he/she should provide any follow-up.

A Word From Verywell

Red cell distribution width (RDW) is a valuable tool in evaluating the different types of anemia and may have a wide range of uses even when a person's red blood cell count is normal. In addition to the conditions mentioned above, some argue that RDW could be a measure of general well-being.

For now, it's uncertain the value this test will have in many conditions but it's noteworthy that simple tests such as these—that can easily be overlooked—may provide important information to be heeded.

Frequently Asked Questions

  • What is RDW in a blood test?

    In a blood test, the red cell distribution width (RDW) is the range of volume and size of the blood cells. It is used to help diagnose certain conditions like anemia, thalassemia, heart disease, diabetes, liver disease, colorectal cancer, and more.

  • What does a high RDW indicate?

    A high RDW has been associated with some types of anemia, including iron deficiency anemia, vitamin B12 and folate deficiency, mixed anemias, sickle cell disease, myelofibrosis, and cold agglutinin disease. It has also been linked to certain conditions unrelated to blood, such as sleep apnea and lupus. A high RDW alone cannot diagnose these conditions, however.

  • Do people with anemia bruise easily?

    Some people with anemia may bruise easily. Other symptoms of anemia can include dizziness, headache, heightened thirst, irritability, sore tongue, lower leg cramping during exercise, tiredness, pale skin, and weakness.

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  1. MedlinePlus. RDW (red cell distribution width). Reviewed July 31, 2020.

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