Coombs Test is a laboratory test used to diagnose red blood cell (RBC) based autoimmune diseases such as autoimmune hemolytic anemia, warm autoimmune hemolytic anemia, and cold agglutinin syndrome. The test is also used to determine if a person has been sensitized to RBCs before receiving a blood transfusion. Additionally, the Coombs test may be used to diagnose hemolytic disease of the newborn (HDN).