Uses, Procedure, and Results

By Lynne Eldridge, MD 

 Medically reviewed by Doru Paul, MD

 Fact checked by Heather Mercer

The red cell distribution width (RDW) blood test measures how equal your red blood cells are in size and shape.

One of several tests included in a complete blood count (CBC), RDW is measured as a percentage. The normal range for RDW in adults is 12% to 15%, however, the reference range can vary based on the lab.

If your RDW blood test is high, it means there is a greater variation in the size of your red blood cells. A high RDW may be a sign of anemia or another nutrient deficiency.

This article covers why RDW blood tests are done, how the results are interpreted, and what to expect if you are having an RDW blood test.

David Silverman / Getty Images

When Is an RDW Blood Test Needed?

The RDW blood test is commonly performed both for screening healthy individuals and to evaluate a wide range of medical conditions, including anemia. It is also known as erythrocyte distribution width or RDW-SD (standard deviation test).

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

• When symptoms of anemia, such as lightheadedness or fatigue, are present

• To help diagnose the causes of anemia (a wide variation in the size of cells or a high RDW may occur with more than one type of anemia)

• To screen people who have a history of a red blood cell condition such as thalassemia

• 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 (an elevated RDW is a strong predictor for eventual heart failure)

RDW results are considered one piece of the diagnostic puzzle. A CBC includes several indices, or measurements, pertaining to your red blood corpuscles (the medical term for "cells").

The other red cell indices include:

• Mean corpuscular volume (MCV): A test that measures the average size of your red blood cells

• Mean corpuscular hemoglobin (MCH): A test that measures the amount of hemoglobin that each red blood cell contains. Hemoglobin is the protein in red blood cells that carries oxygen.

• Mean corpuscular hemoglobin concentration (MCHC): A test that determines the amount of hemoglobin present relative to the size of the red blood cell (hemoglobin concentration)

A CBC also includes indices related to your white blood cells and platelets. RDW test results will be considered alongside other CBC results as a healthcare provider evaluates your situation.

Differentiating Types of Anemia

The results of an RDW blood test are used in combination with other indices to tell one type of anemia from another.

For example, both iron deficiency anemia and thalassemia are usually associated with a low MCV, but the two conditions are treated differently.

Checking the RDW can help distinguish between these two types of anemia, where a high RDW indicates a possible iron deficiency and a normal RDW would point to thalassemia.

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, while non-megaloblastic have 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.

Other Health Concerns

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%) were 71% 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.

Research on 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

• Surgery: Studies looking at different types of surgery have found that RDW may predict the risk of complications after surgery; 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.

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

What Are the Risks of a RDW Blood Test?

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

There are a few limitations to this blood test:

• Anticoagulant agents: Agents used to prevent the blood sample from coagulating can alter the results. RDW should be measured using citrated blood, which uses sodium citrate as an anticoagulant. The citrated test tube has a light blue top. If a lab uses ethylenediaminetetraacetic acid (EDTA) instead, the RDW will be falsely high. The EDTA test tube has a lavender top.

• MCV error: Since the RDW is calculated using MCV, a mistake in MCV will result in an error in the RDW.

• Transfusions: Blood drawn too soon after a blood transfusion is not an accurate reflection of a person's true RDW

Preparing for an RDW Test

No special preparation is needed for an RDW test. Here is what to expect before, during, and after the blood draw.

Before an RDW 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 in many clinics and labs.

Before drawing your blood, a lab technician will clean the area (usually an arm) with an antiseptic and apply a tourniquet to make the vein easier to see. They 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 is 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 to be aware of 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 by applying pressure; 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. If you notice redness, pain, or warmth in the area, contact your healthcare provider.

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 the results when they are available.

Interpreting RDW Test Results

RDW results appear on a CBC report alongside all of the other indices. The combination of results is usually used rather than the RDW alone.

Normal red blood cells average between 6 and 8 micrometers in diameter. RDW results are given as a percentage.

The normal range for RDW varies by laboratory due to differences in equipment but is usually around 12% to 15%. Some lab references ranges go as low as 11.5% or as high as 15.4%.

Here's a sense of the possible reasons why your RDW may be low, high, or normal.

Low RDW

Having a low RDW is normal and ideal. It means that your red blood cells are all uniform in size and shape. A low RDW is not associated with any blood disorder.

High RDW

A few types of anemias associated with an elevated RDW include these situations:

• Iron deficiency anemia

• Anemia caused by vitamin B12 and folate deficiency

• Mixed anemias

• Myelofibrosis (bone marrow cancer that causes anemia)

• Cold agglutinin disease (autoimmune anemia)

• Hereditary red blood cell disorders, such as thalassemia and sickle cell anemia

• Anemia caused by chronic inflammation and autoimmune disorders

Other possible causes of high RDW levels include:

• Chronic liver disease

• Colorectal cancer

• Diabetes

• Heart disease

• Kidney disease

Normal RDW

Even if your CBC results reveal a normal RDW, you can still have anemia. In this case, your healthcare provider will consider other values on your CBC to determine the type of anemia present.

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

• Acute blood loss, hemorrhage

• Thalassemia (some types)

• Anemia caused by chronic diseases such as diabetes, liver, heart, or kidney disease

• Alcohol abuse

• Aplastic anemia (bone marrow cannot produce enough RBC)

RDW + MCV Results

The MCV is of particular interest, as certain RDW and MCV result pairings are indicative of certain diagnoses.

The following are examples of conditions that combined RDW and MCV rates may indicate. Note, however, 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.

Other Tests

RDW results will be considered in the context of additional indices you'll find on your CBC report. In addition to the aforementioned MCV, MCH, and MCHC, these include:

• Red blood cell (RBC) count

• White blood cell (WBC) count

• Hemoglobin and hematocrit count

• Mean platelet volume (MPV)

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

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.

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 (low reticulocyte count) and those in which there is a loss or breakdown of red blood cells (blood loss or hemolysis) that are usually associated with a high reticulocyte count.

• Iron Studies: Serum iron and iron-binding capacity and/or serum ferritin can measure the amount of iron in red blood cells and how much iron is stored in the body in the form of ferritin.

• Vitamin B12 and Folate: If either vitamin B12 or folate deficiency is suspected, a blood level for each will be drawn.

• Hemoglobin Electrophoresis: This study can find sickle cell and some types of thalassemia.

• Bone Marrow Study: A bone marrow aspiration and/or biopsy may be done to look at the RBCs, WBCs, and platelets in the bone marrow.

Follow-Up

Follow-up testing, if the RDW is abnormal, will depend on many factors. Discuss your results with your healthcare provider, and they should provide any follow-up.

How to Lower a High RDW

If your RDW blood test is high, addressing the underlying cause can help to lower the variation in red blood cell size.

Depending on the cause, your healthcare provider may recommend you take the following actions: 

• Eat more foods rich in iron, folate, or vitamin B-12, or take supplements

• Get seven to eight hours of sleep a night19

• Increase the amount of exercise you get20

• Quit smoking21

• Stop drinking alcohol

Summary

Red cell distribution width (RDW) is a valuable tool in evaluating the different types of anemia. It 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.22

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

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Additional Reading

McPherson RA, Pincus MR, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Elsevier; 2021.

By Lynne Eldridge, MD
 Lynne Eldrige, MD, is a lung cancer physician, patient advocate, and award-winning author of "Avoiding Cancer One Day at a Time."