• Also Known As:
  • Red Cell Distribution Width
  • RDW-SD (Standard Deviation) Test
  • Erythrocyte Distribution Width
Medically Reviewed by Expert Board.

This page was fact checked by our expert Medical Review Board for accuracy and objectivity. Read more about our editorial policy and review process.

This article was last modified on
Learn more about...

Test Quick Guide

Red cell distribution width (RDW) is a measurement that describes the variation in the sizes of red blood cells in a sample of blood. Red blood cells (RBCs) carry oxygen throughout the body. The size of RBCs can affect their ability to deliver oxygen to tissues and organs.

The RDW test is one part of a panel of tests called the RBC indices, which help to define different physical characteristics of red blood cells. The RDW test can be used to diagnose and classify conditions like anemia, diabetes, and heart disease.

About the Test

Purpose of the test

The purpose of the RDW test is to assess the range of sizes of red blood cells in a sample of your blood. An RDW test is performed in combination with other tests to diagnose and determine the cause of anemia, a condition in which there are not enough healthy red blood cells to carry enough oxygen to the rest of your body.

The RDW test is part of a complete blood count (CBC), a routine test that is used to diagnose and monitor a wide variety of health conditions. In a CBC report, RDW is grouped with a panel of tests called the red blood cell indices that provide information about certain features and qualities of red blood cells. The RBC indices include:

Collectively, the RBC indices play an important role in diagnosing and determining the underlying cause of anemia. An RDW test can be relevant when evaluating a patient for the certain conditions that may cause anemia including:

  • Iron deficiency
  • Vitamin B12 or folate deficiency
  • Thalassemia, an inherited blood disorder
  • Heart disease
  • Cancer
  • Kidney disease
  • Liver disease

What does the test measure?

The RDW test measures the variation in the sizes of your red blood cells or whether red blood cells are more similar or less similar according to their size. In this case, width does not refer to the actual size of the red blood cell. Rather, RDW is a calculation of the distance between the range of red blood cell sizes as they are displayed on a graph called a histogram.

Blood cells that are similar in size will appear as data points on a histogram that are clustered closely together, which will result in a lower RDW. When there is a greater variation of blood cell size, data points will appear to have a wider distribution across the graph and the RDW is higher.

When should I get an RDW test?

RDW is part of a CBC, a common test that analyzes the quality and condition of your blood cells. You may have a CBC test conducted as part of a routine health exam or if you are being assessed for a health condition.

The RDW test results are usually compared with other tests that evaluate red blood cell function in order to check for signs of anemia. Anemia is a common condition in which your body does not have enough healthy red blood cells and can be caused by a wide variety of health problems. Symptoms of anemia can be mild or severe depending on the underlying condition or disorder.

Mild anemia may present with no signs and symptoms, develop slowly over time, or occur suddenly. Some signs of early or mild anemia may include:

  • Headaches
  • Loss of appetite
  • Feeling unusually weak or tired, including excessive fatigue after exercise
  • Numbness or tingling in your hands and/or feet
  • Problems with concentrating or thinking
  • Irritability or feeling agitated

As the anemia progresses, additional signs and symptoms may include:

  • Ulcers of the mouth
  • Being short of breath at rest or mild activity
  • Unusually pale skin
  • An unusually red or possibly sore tongue
  • Feeling dizzy
  • Brittle nails
  • Desire to eat dirt, ice, or other non-food items
  • Blue color in the whites of the eyes

Finding an RDW Test

How to get tested

The RDW test is part of a complete blood count. A blood draw is generally required for the CBC to be accurate and is most often conducted by a licensed professional in a health care setting.

Can I take the test at home?

CBC testing is available as an at-home test and can provide results for RDW and other RBC indices. At-home testing kits include instructions and equipment for collecting a blood sample.

Once complete, the blood sample can be sent to a laboratory for analysis. Results of at-home tests are generally ready in about two to three business days after the sample arrives at the lab. Results may be reported through an online health portal or via email.

Diagnosis or monitoring of a health condition cannot be performed using at-home CBC testing. For an accurate interpretation of your test results, results must be shared with a health care provider who is familiar with your medical history and condition.

To confirm results from the at-home test, your health care provider may recommend retesting using a standard blood draw.

How much does the test cost?

The cost of a CBC, which includes RDW results, depends on several factors, including whether you have health insurance coverage and where the test is conducted. Additionally, the cost may include copays and/or deductibles.

For patients without health insurance, or for whom insurance doesn’t cover the cost of testing, it may be helpful to discuss the cost of RDW testing with a doctor or hospital administrator.

Taking the RDW Test

The RDW test is part of the complete blood count, which is performed by a licensed professional. A blood sample is drawn from a vein, usually from the inside crook of your elbow or the top of your hand, into a sample tube.

Before the test

No preparation is required before a blood draw for a CBC unless specified by your doctor.

During the test

If you are having your blood drawn for an RDW in a medical facility, there are several steps that you can expect:

  1. A health care provider will locate the vein in your arm or hand from which they will draw blood.
  2. An alcohol wipe is used to cleanse your arm where the needle will be inserted.
  3. An elastic band is then placed around your upper arm making the vein more visible and easier to access with the needle.
  4. Once the needle is placed in your vein, a tube is then attached to the needle which then automatically fills with the blood sample. You might feel a pinch or a little pain when the needle is inserted.
  5. Once the test tube or sample vial is filled, the needle and tourniquet are removed, and the test is over. Sample collection typically only takes a few minutes.

After the test

When the blood draw is finished, a bandage or cotton swab will be placed on the area where the needle was inserted. The health care provider may recommend that you keep this in place for an hour or more to help prevent bleeding.

After a blood draw, it is common to experience some dizziness, lightheadedness and/or bruising. Other less common side effects from a blood draw may include persistent bleeding, tingling of hands or feet, nausea, or fainting.

Your health care provider may ask you to stay and be monitored for these side effects for a few minutes after the blood draw to make sure you are able to walk and/or drive safely.

RDW Test Results

Receiving test results

Depending on the laboratory instrumentation and procedures, the results for your red cell distribution width test can be available within a few days of the blood sample arriving at the laboratory.

The report of your RDW test results may be available through an online health portal or provided by email. Hard copies can be provided upon request. Your health care provider can interpret your test results and determine what they mean based on your health history and present condition.

Interpreting test results

RDW is usually reported as a percentage that describes the amount of variability in the size of red blood cells. The RDW result is interpreted by comparing it to a reference range, or a range of values that the laboratory conducting the test has established as an expected RDW for healthy people.

If RDW is within the reference range, it is considered normal; if RDW is outside the reference range, it is considered high or elevated.

Normal RDW means that the red blood cells in the sample that was tested are similar to each other in their size or volume. Normal RDW can occur even when someone has a health problem that causes red blood cells to be abnormally large or small.

Elevated RDW indicates that the sizes of the red blood cells in the sample have a greater degree of variation. Although an elevated RDW is an abnormal result, on its own it does not provide enough information for a physician to make a diagnosis or fully evaluate a patient’s health.

The interpretation of a normal or abnormal RDW result depends on a comparison to other RBC indices, particularly MCV, which is the average size of red blood cells. An interpretation of RBC indices provides information about the size, quality, and function of red blood cells, which can be used to determine if a person has anemia and point to the possible underlying cause.

Are test results accurate?

The RDW test is one part of the complete blood count, one of the most useful and common blood tests in modern medicine. It is generally accurate and precise due to the modern laboratory methods and equipment used to analyze the blood samples.

Normal values for reference ranges may vary slightly between laboratories. Results outside the normal reference range do not necessarily indicate the presence of a medical condition.

Do I need follow-up tests?

Depending on the results of the rest of your CBC, both normal and abnormal RDW test results may require follow-up testing. Additional blood testing, other laboratory tests, and imaging tests may all be considered based on the outcome of your initial RDW.

Many factors are considered when determining what follow-up testing is needed. The decision is based on your specific situation, including your symptoms, medical history, and the results of other tests. For questions about follow-up testing, make sure to speak with your health care provider.

Questions for your doctor about test results

Your RDW levels may have implications for your health. If you discuss your results with your doctor, here are some questions you may want to ask:

  • What do my RDW levels indicate about my health?
  • Can any diagnosis be made based on my red blood cell indices?
  • Will I need any follow-up tests based on my RDW results?
  • If my test results are abnormal, is there anything I can do to improve my health?

Related Tests

RDW test vs. complete blood count

The RDW is a measurement of the variability in size of your red blood cells. This measurement is a standard part of the complete blood count, a collection of tests that allow your health care provider to analyze the cells in your blood. When interpreted alongside the rest of the CBC, your RDW results can help your doctor understand the status of your health.

Other parts of the CBC that appear with your RDW results include:

  • Red blood cells (RBC)
  • White blood cells (WBC)
  • Platelets (Plt)
  • Mean corpuscular hemoglobin (MCH)
  • Mean corpuscular hemoglobin concentration (MCHC)
  • Mean corpuscular volume (MCV)
  • Hematocrit (Hct)
  • Hemoglobin (Hgb)

RDW test vs. other red blood cell indices

RDW is often included as part of a collection of test results in the complete blood count called the red blood cell (RBC) indices. The RBC indices are used to assess certain qualities about the size, function, and performance of red blood cells. The RDW test is a way of measuring the variation in the size of red blood cells. Other RBC indices include:

  • MCV, which is the average size of red blood cells
  • MCH, which is the average amount of hemoglobin in an individual red blood cell
  • MCHC, which is a measure of the hemoglobin in red blood cells as it relates to the size of the cell

View Sources

A.D.A.M. Medical Encyclopedia. Blood smear. Updated January 13, 2020. Accessed November 12, 2021. https://medlineplus.gov/ency/article/003665.htm

A.D.A.M. Medical Encyclopedia. RBC count. Updated January 13, 2020. Accessed October 29, 2021. https://medlineplus.gov/ency/article/003644.htm

A.D.A.M. Medical Encyclopedia. RBC indices. Updated January 13, 2020. Accessed September 27, 2021. https://medlineplus.gov/ency/article/003648.htm

A.D.A.M. Medical Encyclopedia. Anemia. Updated February 6, 2020. Accessed September 27, 2021. https://medlineplus.gov/ency/article/000560.htm

American Board of Internal Medicine. ABIM laboratory test reference ranges. Updated July 2021. Accessed October 3, 2021. https://www.abim.org/Media/bfijryql/laboratory-reference-ranges.pdf

Braunstein EM. Approach to the patient with anemia. Merck Manuals Professional Edition. Updated September 2020. Accessed October 29, 2021. https://www.merckmanuals.com/professional/hematology-and-oncology/approach-to-the-patient-with-anemia/evaluation-of-anemia

Camaschella C, Weiss G. Anemia of chronic disease/anemia of inflammation. In: Mentzer WC, Means RT, eds. UpToDate. Updated May 18, 2020. Accessed December 8, 2021. https://www.uptodate.com/contents/anemia-of-chronic-disease-anemia-of-inflammation

Camaschella C. Microcytosis / microcytic anemia In: Mentzer WC, Means, Jr RT Jr , eds. UpToDate. Updated October 19, 2021. Accessed November 12, 2021. https://www.uptodate.com/contents/microcytosis-microcytic-anemia

Center for Disease Control. Blood and urine collection. Accessed Oct 25, 2021. https://www.cdc.gov/nchs/data/nhanes/nhanes_09_10/labcomp_f.pdf

Curry CV. Red cell distribution width (RDW) test. In: Staros EB, ed. Updated January 12, 2015. Access December 7, 2021. https://emedicine.medscape.com/article/2098635-overview

George TI. Automated hematology instrumentation. In: Uhl L, ed. UpToDate. Updated June 3, 2021. Accessed October 29, 2021. https://www.uptodate.com/contents/automated-hematology-instrumentation

Kuter DJ. Laboratory tests for blood disorders. Merck Manuals Consumer Edition. Updated June 2021. Accessed September 27, 2021. https://www.merckmanuals.com/home/blood-disorders/symptoms-and-diagnosis-of-blood-disorders/laboratory-tests-for-blood-disorders

Means, RT Jr, Brodsky RA. Diagnostic approach to anemia in adults. In: Mentzer WC, ed. UpToDate. Updated June 10, 2021. Accessed October 29, 2021. https://www.uptodate.com/contents/diagnostic-approach-to-anemia-in-adults

MedlinePlus: National Library of Medicine. RDW (red cell distribution width). Updated July 31, 2020. Accessed November 12, 2021. https://medlineplus.gov/lab-tests/rdw-red-cell-distribution-width/

MedlinePlus: National Library of Medicine. Red blood cell (RBC) count. Updated October 4, 2021. Accessed November 12, 2021. https://medlineplus.gov/lab-tests/red-blood-cell-rbc-count/

MedlinePlus: National Library of Medicine. Red blood cell (RBC) indices. Updated October 5 2021. Accessed October 29, 2021. https://medlineplus.gov/lab-tests/red-blood-cell-rbc-indices/

National Heart, Lung, and Blood Institute. Blood tests. Date unknown. Accessed September 12, 2021. https://www.nhlbi.nih.gov/health-topics/blood-tests

Rosenthal DS. Evaluation of the peripheral blood smear. In: Brodsky RA, ed. UpToDate. Updated May 17, 2021. Accessed October 29, 2021. https://www.uptodate.com/contents/evaluation-of-the-peripheral-blood-smear

Ask a Laboratory Scientist

Ask a Laboratory Scientist

This form enables patients to ask specific questions about lab tests. Your questions will be answered by a laboratory scientist as part of a voluntary service provided by one of our partners, American Society for Clinical Laboratory Science. Please allow 2-3 business days for an email response from one of the volunteers on the Consumer Information Response Team.

Send Us Your Question