The indirect antiglobulin (indirect Coombs) test is used to detect IgG antibodies against red blood cells (RBCs) in a patient's plasma. The patient's plasma is incubated with reagent RBCs; then Coombs serum (antibodies to human IgG, or human anti-IgG) is added. If agglutination occurs, IgG antibodies (autoantibodies or alloantibodies) against RBCs are present. This test is also used to determine the specificity of an alloantibody.