The direct and indirect Coombs tests, also known as antiglobulin test (AGT), are blood tests used in immunohematology. The direct Coombs test detects antibodies that are stuck to the surface of the red blood cells. Since these antibodies sometimes destroy red blood cells they can cause anemia; this test can help clarify the condition. The indirect Coombs test detects antibodies that are floating freely in the blood. These antibodies could act against certain red blood cells; the test can be carried out to diagnose reactions to a blood transfusion.
The direct Coombs test is used to test for autoimmune hemolytic anemia, a condition where the immune system breaks down red blood cells, leading to anemia. The direct Coombs test is used to detect antibodies or complement proteins attached to the surface of red blood cells. To perform the test, a blood sample is taken and the red blood cells are washed (removing the patient's plasma and unbound antibodies from the red blood cells) and then incubated with anti-human globulin ("Coombs reagent"). If the red cells then agglutinate, the test is positive, a visual indication that antibodies or complement proteins are bound to the surface of red blood cells and may be causing destruction of those cells.
The indirect Coombs test is used in prenatal testing of pregnant women and in testing prior to a blood transfusion. The test detects antibodies against foreign red blood cells. In this case, serum is extracted from a blood sample taken from the patient. The serum is incubated with foreign red blood cells of known antigenicity. Finally, anti-human globulin is added. If agglutination occurs, the indirect Coombs test is positive.
Mechanism
The two Coombs tests are based on anti-human antibodies binding to human antibodies, commonly IgG or IgM. These anti-human antibodies are produced by plasma cells of non-human animals after immunizing them with human plasma. Additionally, these anti-human antibodies will also bind to human antibodies that may be fixed onto antigens on the surface of red blood cells (RBCs). In the appropriate test tube conditions, this can lead to agglutination of RBCs and allowing for visualisation of the resulting clumps of RBCs. If clumping is seen, the Coombs test is positive; if not, the Coombs test is negative.
Common clinical uses of the Coombs test include the preparation of blood for transfusion in cross-matching, atypical antibodies in the blood plasma of pregnant women as part of antenatal care, and detection of antibodies for the diagnosis of immune-mediated hemolytic anemias.
Coombs tests are performed using RBCs or serum (direct or indirect, respectively) from venous whole blood samples which are taken from patients by venipuncture. The venous blood is taken to a laboratory (or blood bank), where trained scientific technical staff do the Coombs tests. The clinical significance of the result is assessed by the physician who requested the Coombs test, perhaps with assistance from a laboratory-based hematologist.
Direct Coombs test
The direct Coombs test, also referred to as the direct antiglobulin test (DAT), is used to detect if antibodies or complement system factors have bound to RBCs surface antigens. The DAT is not required for pre-transfusion testing but may be carried out by some laboratories. Before transfusion, an indirect Coombs test is often done.
Uses
The direct Coombs test is used clinically when immune-mediated hemolytic anemia (antibody-mediated destruction of RBCs) is suspected. A positive Coombs test indicates that an immune mechanism is attacking the patient's RBCs. This mechanism could be autoimmunity, alloimmunity or a drug-induced immune-mediated mechanism.
Examples of alloimmune hemolysis
Hemolytic disease of the newborn (also known as HDN or erythroblastosis fetalis) Rh D hemolytic disease of the newborn (also known as Rh disease) ABO hemolytic disease of the newborn (the direct Coombs test may only be weakly positive) Anti-Kell hemolytic disease of the newborn Rh c hemolytic disease of the newborn Rh E hemolytic disease of the newborn Other blood group incompatibility (RhC, Rhe, Kidd, Duffy, Lewis, MN, P and others) Alloimmune hemolytic transfusion reactions
Examples of autoimmune hemolysis/immunohemolytic hemolysis
Warm antibody autoimmune hemolytic anemia
Idiopathic
Systemic lupus erythematosus
Evans' syndrome (antiplatelet antibodies and hemolytic antibodies) Cold antibody immunohemolytic anemia Idiopathic cold hemagglutinin syndrome Waldenström's macroglobulinemia Infectious mononucleosis Paroxysmal cold hemoglobinuria (rare)