Red Blood Cell Distribution Width: Uses, Side Effects, Procedure


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

However, even when blood counts, such as the red blood cell count, are normal, the RDW can be a valuable test. For example, it can predict iron deficiency in pregnant women even before anemia develops (iron deficiency increases the risk for both mother and baby). It can also be helpful in assessing the risk of heart disease or cancer, and some health professionals believe it could be a test that assesses general well-being.

There are limitations in the evaluation of RDW, for example, after a blood transfusion. RDW can also be called red blood cell distribution width or RDW-SD (standard deviation test).

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Red blood cell distribution width (RDW) is performed as part of a complete blood count and is therefore a frequently performed test used both to detect healthy people and to evaluate a wide range of diseases.

There are times when healthcare providers may pay specific attention to the value of RDW:

  • with symptoms of anemia such as dizziness or fatigue
  • to help diagnose causes of anemia (there may be a wide variation in cell size or a high RDW if there is more than one type of anemia)
  • to screen for people who have a history of red blood cell disease such as thalassemia
  • with heart disease (increased RDW is a strong predictor of possible heart failure)
  • for screening for early iron deficiency in pregnant women before anemia
  • to detect vitamin B12 and folate deficiency early before other signs are noticed in the blood
  • get an idea of when more blood tests (peripheral blood smear) are needed
  • as an adjunct to assess disease risk (heart disease, cancer, etc.) or determine prognosis


RDW can be represented as standard deviation (SD) or coefficient of variation (CV), but RDW-CV is the most common. A standard deviation of the red blood cell volume divided by MCV times 100.

  • SD / MCV x 100


RDW is used to describe the rate of change in the size of red blood cells, and the term anisocytosis is used to describe this change. In other words, if we talk about significant anisocytosis in a blood smear, this would mean that the red blood cells vary significantly in size.

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


If the RDW is compiled after a blood transfusion, it will not accurately reflect the RDW of human cells. If the lab uses blood with EDTA anticoagulant instead of citrated blood, the reading will be falsely high. Since RDW-CV is calculated using MCV, an error in MCV will result in an error in RDW.

Similar tests

Changes in the size of red blood cells can also be seen visually by looking at the morphology of a peripheral blood smear, although this test is usually done after a complete blood count to detect abnormalities.

Additional exams

Since RDW runs as part of CBC, the number is listed along with several other values, and a combination of results is generally used, not just RDW. These include the number of blood cells of each type and other indicators of red blood cells .

  • Erythrocytes (erythrocytes)
  • Leukocytes (leukocytes)
  • Platelets
  • Hemoglobin and hematocrit
  • Mean corpuscular volume (MCV) or a measure of the size of red blood cells
  • Mean corpuscular hemoglobin concentration (MCHC) or a measure of the hemoglobin concentration in a given volume of red blood cells
  • Average corpuscular hemoglobin (MCH), which corresponds to MCV and generally has little value
  • The mean platelet volume (MPV) , which is the mean platelet volume, can provide clues to many diseases.

Additional exams

In addition to the complete blood count, other tests that may be ordered to evaluate anemia include reticulocyte counts, blood smears to determine morphology, iron tests, and more.

Risks and contraindications.

Since RDW is part of a simple blood test, there are very few risks. In rare cases, people may have bleeding from the puncture site, bruising (hematoma), or infection.

Before the test

Before running RDW (CBC), there are no particular power or activity restrictions. You will need an insurance card and it will be helpful to provide your healthcare provider with the results of your previous blood tests for comparison.

During the exam

A complete blood count can be done both in the hospital and in many clinics. Before drawing blood, the technician will clean the affected area (usually the arm) with an antiseptic and apply a tourniquet to make the vein easier to see. Then the needle will be inserted through the skin into a vein. You may feel a sharp (but short) bite when inserting the needle, and some people may feel dizzy or weak.

Once the sample is removed, the needle is withdrawn and pressure is applied to the puncture wound. A bandage is then applied to keep it clean and reduce bleeding.

After the test

Once you have blood, you can go home. Possible side effects can include:

  • Bleeding Occasionally, the area where the blood was drawn will continue to bleed, although this is more common in people taking blood thinners or with a bleeding disorder. Most of the time, this can be resolved with pressure, but if the bleeding persists, you should see your doctor.
  • Hematoma. In rare cases, a large bruise can form at the site of the blood sample. This, again, is more common in those who take blood thinners, such as antiplatelet drugs.
  • Infection. There is a very small risk that an infection will develop as a result of bacteria entering the skin into the body during blood sampling.

interpretation of results

If there is a laboratory associated with your clinic, most of the time you will receive the results shortly after they are completed. In some cases, a blood sample will be sent to a lab and your healthcare provider will call and report the results when they are available.

When you get your results, it helps to know the exact numbers, including your RDW. As discussed below, RDW can provide important information even if the rest of your CBC tests are normal.

Control range

Reference ranges for RDW may vary slightly depending on the laboratory performing the test. Normal red blood cells have an average diameter of 6 to 8 microns. RDW estimates the change in cell size and is expressed as a percentage. The normal range for RDW is about 11.8 to 15.6 percent, and this number often increases with age.

Normal RDW with anemia

Some examples of anemias in which RDW is usually normal are:

  • Thalassemia (some types)
  • Chronic disease anemia
  • Liver disease
  • Alcoholic anemia
  • Aplastic anemia

RDW high

Several types of anemia associated with an increase in RDW include:

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

Sharing RDW and MCV

The use of a combination of RDW and MCV is very helpful in differentiating some types of anemia that would otherwise be difficult to distinguish from each other. For example, both iron deficiency anemia and thalassemia are generally associated with a low MCV (microcytic anemia), but the two conditions are treated differently. Checking the RDW can help you distinguish between the two.

Similarly, megaloblastic anemias (such as vitamin B12 and folate deficiencies) and non-megaloblastic anemias (such as anemia associated with liver disease) are associated with high MCV (macrocytic anemias), but are again treated differently. In this case, megaloblastic anemias usually have a high RDW and non-megaloblastic anemias have a low RDW, which helps to distinguish.

RDW can also be very helpful for 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.

Below are examples of conditions that may indicate RDW and MCV rates. It is important to note that there are exceptions to these general rules; for example, sometimes anemia in chronic disease is associated with low MCV and sometimes iron deficiency anemia shows normal MCV.

  • High RDW and low MCV: iron deficiency anemia, sickle cell anemia, beta thalassemia, or hemoglobin H
  • High RDW and normal MCV: early iron deficiency anemia, early B12 / folate deficiency, (chronic) blood loss, or hemolysis
  • High RDW and high MCV: vitamin B12 deficiency, folate deficiency, immune hemolytic anemia, or this combination is common in newborns.
  • Normal RDW and low MCV: anemia of chronic disease, thalassemia, hemoglobin E.
  • Normal RDW and normal MCV: blood loss (acute), anemia due to kidney disease, some abnormal hemoglobins, or spherocytosis
  • Normal RDW and high MCV: aplastic anemia, liver disease, alcohol abuse, some drugs cause this combination (such as chemotherapy or antiviral drugs)

Other tests

In addition to a complete blood count, other tests that may be done to detect anemia include:

  • Reticulocyte count: The reticulocyte count helps classify anemias into anemias based on insufficient production of red blood cells (normal reticulocyte count) and anemias in which there is loss or destruction of red blood cells (loss of blood or hemolysis), which they are generally associated with high levels of red blood cells. reticulocyte counts ..
  • Blood smear: In a peripheral blood smear, a sample of blood is seen under a microscope. In addition to being able to visualize differences in size and shape, other results may include target cells, nucleated red blood cells, fragmented red blood cells (with hemolysis), and more.
  • Iron Studies: Serum iron and serum iron and / or ferritin binding capacity can determine the body's iron stores.
  • Vitamin B12 – If a vitamin B12 deficiency is suspected, vitamin B12 levels will be determined.
  • Hemoglobin electrophoresis : This test can detect some (but not all) types of thalassemia.
  • Bone marrow examination: A bone marrow aspiration and / or biopsy may be performed to examine the types of cells in the bone marrow and iron stores.

Use without anemia for RDW

The RDW can be a very useful number, even if there are no signs of anemia (if the red blood cell count and hemoglobin level are normal).

RDW can predict the overall risk of death in people over 45 (people with high RDW are more likely to die earlier than those with lower RDW).

In recent years, there have been many studies examining 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 in people with high blood pressure. A 2014 study found that people with very high RDWs (in the top 5 percent) were 71 percent more likely to have a heart attack than those with lower RDWs. A high RDW can also help predict the risk of heart disease in people infected with HIV.
  • Cancer: Research has looked at RDW's role in cancer development in different ways. For several types of cancer (such as blood cancer, lung cancer, and colon cancer), a high RDW can mean a worse prognosis.

On the other hand, researchers have examined the potential of RDW to predict cancer risk in people who do not currently have the disease. For example, they found a dose-dependent relationship between high RDW values in postmenopausal men and women and future cancer risk.

For people screened for unintentional weight loss , a high RDW increased the likelihood that the weight loss was caused by cancer.

  • Surgery : Research on various types of surgery has shown that RDW can predict the risk of complications after surgery, to the extent that RDW has been suggested to be important to orthopedic surgeons.
  • Sleep – A high RDW is associated with some sleep disorders, such as sleep apnea, and is also increased in those who sleep too little or too much or work shifts.
  • Diabetes – People with elevated RDW appear to be at increased risk of developing diabetes.

This area of research (exploring the role of RDW in the assessment of conditions other than blood) is fairly new and it is hoped that more information will be available to better understand the potential benefits of using RDW in the future.

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

Follow up

Further 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 action.

Get the word of drug information

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

It is not known at this time how important this test will be in many settings, but it is noteworthy that simple tests like these, which can easily be overlooked, can provide important information to consider.

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