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Uses of the Red Cell Distribution Width Test in Medical Science

written by: Cordie Kellerman•edited by: Emma Lloyd•updated: 10/27/2009

The RDW blood test is one of several components of a CBC (complete blood count). Red cell distribution width measures the variance in the sizes of the red blood cells in a blood sample. It is useful in the diagnosis of several types of anemia.

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    One of the blood tests most commonly ordered by physicians is the CBC, or complete blood count. The CBC is actually a set of tests that analyze several different components of blood. One of those components is the red cell distribution width, abbreviated RDW. The RDW test measures variance in the sizes of red blood cells.

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    Normal Results

    Typically, red blood cells are fairly uniform in size and a normal RDW will be a low number (11.5 to 14.5). If there is a large variance in the size of red blood cells the RDW will be elevated (greater than 14.5). This may be indicative of any of several medical conditions.

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    Why Would My Doctor Order a RDW Blood Test?

    Your doctor may order a CBC, which will include the RDW index, simply as a measure of your general health. It may be used to screen for developing problems or as a baseline to which future tests can be compared. Your doctor may also order it if he or she suspects certain specific disorders based on your symptoms or medical history. Your doctor may be especially interested in the RDW index if anemia is indicated. The RDW index is very useful in determining the cause of the anemia.

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    What Should I Expect During the Procedure?

    The RDW test requires a blood sample. This is usually drawn by inserting a needle into a vein and collecting the blood in a glass vial. Most often a vein from an arm (inside elbow) is used. Sometimes a different site, like the back of a hand, will work better. For young children and infants, a finger or heel is stuck with a sharp instrument and made to bleed. The blood is collected into a glass tube (pipette).

    A blood draw is slightly uncomfortable but should not be a source of great distress. Usually it involves only the momentary sting of the needle poking through the skin. After the blood is drawn the site may bruise and be somewhat tender. To minimize bruising, apply pressure to the site for several minutes or until bleeding stops, then apply ice to the site throughout the day.

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    What Do the Results of the Test Mean?

    The most common cause of an elevated RDW is anemia. There are many causes of anemia but in general it is the result of not having enough red blood cells (RBC). The body responds to the deficit by increasing its production of new red blood cells. The new, immature RBCs (also called reticulocytes) are larger than mature RBCs. The presence of both mature RBCs and a large number of reticulocytes causes a greater than normal variance in the size of RBCs.

    To determine the exact cause of an elevated RDW, it is often necessary to also look at the MCV, which is another component of the CBC. MCV is the abbreviation for mean corpuscular volume and it measures the mean red blood cell volume, or the average size of the red blood cells in the sample. Different disorders are indicated when both RDW and MCV are elevated as opposed to one being elevated and the other normal or lower than normal.

    Possible causes of an elevation of both RDW and MCV are hemolytic anemia, or anemia caused by a deficiency in either vitamin B12 or folate, or liver disease. On the other hand, if RDW is elevated but MCV is lower than normal, iron deficiency anemia will be suspected. Other possible disorders with these findings would Thalassemia or RBC fragmentation. If the RDW index is in the normal range, but MCV is elevated, then your doctor may do further testing for aplastic anemia or possibly pre-Leukemia.