A high RDW blood test result may indicate anemia due to either iron deficiency or a deficiency of folate and vitamin B12, according to Wikipedia. It may also be a result of a recent hemorrhage. False high RDW readings sometimes occur when the wrong type of blood is used for the test. As MedFriendly explains, other causes for a high RDW test result include liver disease and hemolytic anemia.
An RDW test is a test of red cell distribution width, and it measures the variation in the size of red blood cells. According to MedFriendly, this test is often done in conjunction with measuring the mean corpuscular volume, or MCV, of the red blood cells. The MCV is the average amount of space that each cell takes up within the bloodstream.
A comparison of the RDW and MCV measurements helps to determine what type of anemia a person is experiencing. When anemia is present and RDW and MCV are both high, anemia is typically due to folate and vitamin B12 deficiency. As Health Research Funding explains, red blood cells increase in size due to B12 and folate deficiency, raising the MCV. When RDW levels are high but MCV levels are not, the cause is more likely iron-deficiency anemia. In iron-deficiency anemia, a lack of iron in the blood causes a decrease in hemoglobin, which is the substance in red blood cells that carries oxygen.