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.

Mechanism

thumb|350px|Schematic showing the direct and indirect Coombs tests

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

500px|thumb|Algorithm for the main diagnoses in a positive DAT

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. Historically, it was done in test tubes. Today, it is commonly done using automated solid phase or gel technology.

References

  • Coombs' test- Medlineplus.org.
  • Drugs that cause haemolytic anemia - Merck Manual.