What Is the Red Blood Cell Distribution Width?

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  • Written By: Andrew Kirmayer
  • Edited By: Shereen Skola
  • Last Modified Date: 04 September 2016
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The condition of red blood cells is often an indicator of an individual’s health. One test that is sometimes performed helps measure red blood cell distribution width (RDW); this can determine how much the cells in a sample vary in size. The measurement generally does not represent physical diameter, however. It typically represents the width of a graphical curve that shows how the volume of the cells varies. A normal range is usually between 11% and 15% in the human body.

Red blood cell distribution width is usually determined mathematically; the formula normally includes the mean corpuscular volume, or the average amount of space that each cell takes up. The mathematical principle of standard deviation is applied to the value, and divided by the average volume. A result is then multiplied by 100 to determine the RDW. Medical laboratories typically have instruments that can pick up the pulses produced by red blood cells; stronger pulses are generally produced by larger cells, while weaker ones usually come from smaller cells.


Lab reports often indicate the normal range for red blood cell distribution width. The ranges, however, can vary depending on what kind of equipment is used for measurement. If the RDW is high on a blood test, it generally means that the cells vary quite a bit in size; anemia caused by an iron deficiency is sometimes a cause. Low levels of vitamin B12 or folic acid often decrease the count as well. The carbon in the vitamins, needed by the body, can be lacking for normal function to take place.

If red blood cell distribution width is high, it may also indicate a blood disorder in which the production of hemoglobin, which carries oxygen, is affected. Other conditions can cause the red blood cells to break up into fragments, which usually makes the RDW value go up. Low red blood cell distribution width, on the other hand, is often an indicator of problems as well. A type of anemia can lower the count if there are an abnormally low number of cells, yet which are relatively large. Deficiencies of certain vitamins can also be the reason for a low RDW percentage.

Red blood cell distribution width can be correlated with the outcomes of other health conditions. A high variability in blood cell size often results in a less optimistic prognosis for people with heart failure, based on studies. The count can represent an insufficient production of new cells as well as more frequent destruction of them than normal, thus complicating underlying medical conditions.


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