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A lower RDW means that your red blood cells are all about the same size.

If the red cell distribution width is high, the sizes of your red blood cells vary more than what's considered normal. 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 a RDW blood test.

The RDW blood test is commonly performed both for screening healthy individuals and to evaluate a wide range of medical conditions, including anemia.

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

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:

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.

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 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 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).

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:

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

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.

RDW may also be referred to as erythrocyte distribution width or RDW-SD (standard deviation test).

There are a few limitations with this blood test. 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 (think of those colored tops on the blood collection tubes). Since the RDW is calculated using MCV, an error in MCV will result in an error in the RDW.

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.

A CBC may be drawn in the hospital as well as many clinics and labs.

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. 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 applied to the puncture wound. A dressing is then applied to keep the area clean and reduce any bleeding.

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

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.

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 is roughly 11.5% to 15.4%, and the number often increases with age, and can vary somewhat by the laboratory doing the test, however.

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:

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:

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

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