An RDW test is a red cell distribution width test, which is one of the parameters measured in blood tests, states Medscape. Higher RDW results mean that the patient's red blood cells have a greater variation in size or volume.
The results from an RDW test may aid medical professionals in diagnosing early iron, folate and B12 deficiencies, as well as sickle cell disease and liver disease, explains Medscape. Additionally, RDW tests help distinguish between anemic disorders, such as between iron deficiency anemia versus heterozygous thalassemia. As with any medical test, RDW tests should be interpreted by a medical professional.