RDW BLOOD TEST. The red blood cell distribution width (“RDW”) test is normally done when the complete blood count is ordered by the health practitioner. The RDW test measures the variations in size of red blood cells. It is used with the MCV (mean corpuscular volume) test results to better determine types of anemia. Normally red blood cells are between six to eight micrometers in diameter (a micrometer is one millionth of a meter and its symbol is μm).