Coombs test

The direct and indirect Coombs tests, also known as antiglobulin test (AGT), are

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.^[2]

Mechanism

The two Coombs tests are based on anti-human

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.^[5] The DAT is not required for pre-transfusion testing^[6] but may be carried out by some laboratories.^{[clarification needed]} 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.^[3]

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)

Drug-induced immune-mediated hemolysis

• Methyldopa (IgG mediated type II hypersensitivity)
• Penicillin (high dose)
• Quinidine (IgM mediated activation of classical complement pathway and Membrane attack complex, MAC)

(A memory device to remember that the DAT tests the RBCs and is used to test infants for haemolytic disease of the newborn is: Rh Disease; R = RBCs, D = DAT.)

Laboratory

The patient's

The indirect Coombs test is used to screen for antibodies in the preparation of

The IAT is a two-stage test. (A cross match is shown visually in the lower half of the schematic as an example of an indirect Coombs test).^[citation needed]

First stage

Nonpatient, washed

The RBCs are washed three or four times with isotonic saline solution and then incubated with antihuman globulin. If antibodies have bound to RBC surface antigens in the first stage, RBCs will agglutinate when incubated with the antihuman globulin (also known Coombs reagent) in this stage, and the indirect Coombs test will be positive.

Titrations

By diluting a serum containing antibodies the quantity of the antibody in the serum can be gauged. This is done by performing serial dilutions of the serum and finding the maximum dilution of test serum that is able to produce agglutination of relevant RBCs.^{[citation needed]}

Coombs reagent

Coombs reagent (also known as Coombs antiglobulin or antihuman globulin) is used in both the direct Coombs test and the indirect Coombs test. Coombs reagent is antihuman

The Coombs test was first described in 1945 by Cambridge immunologists Robin Coombs (after whom it is named), Arthur Mourant and Rob Race.^[8] Historically, it was done in test tubes. Today, it is commonly done using automated solid phase or gel technology.

References