Coombs' test

Coombs' test, also known as the antiglobulin test (AGT), is a clinical blood test used in immunohematology to detect antibodies that act against the surface of red blood cells (RBCs). The test is named after Robin Coombs, a British immunologist who, along with colleagues, developed the test in 1945. The Coombs' test is primarily used to diagnose conditions that lead to the destruction of red blood cells, such as hemolytic anemia, autoimmune diseases, and incompatibilities in blood transfusions.

Types of Coombs' Test

The Coombs' test is divided into two main types: the Direct Coombs' Test (DCT) and the Indirect Coombs' Test (ICT).

Direct Coombs' Test (DCT)

The Direct Coombs' Test is used to detect antibodies that are already bound to the surface of red blood cells in the patient's body. This test is commonly employed to diagnose conditions like autoimmune hemolytic anemia (AIHA), where the body's immune system mistakenly targets and destroys its own RBCs.

Indirect Coombs' Test (ICT)

The Indirect Coombs' Test, on the other hand, is used to screen for unbound antibodies present in the patient's serum. This is particularly useful in pre-transfusion testing, to ensure compatibility between donor and recipient blood, and in prenatal testing, to check for antibodies in a pregnant woman's blood that might cross the placenta and attack the fetus's RBCs, leading to hemolytic disease of the newborn (HDN).

Procedure

The procedure for both DCT and ICT involves mixing the patient's blood sample with a reagent containing anti-human globulin (AHG). If there are antibodies bound to RBCs (in DCT) or antibodies present in the serum that can bind to RBCs (in ICT), the AHG will cause the RBCs to agglutinate. This agglutination is visible to the naked eye and indicates a positive test result.

Clinical Significance

The Coombs' test plays a crucial role in diagnosing and managing various medical conditions. In transfusion medicine, it helps prevent blood transfusion reactions by ensuring compatibility between donor and recipient blood. In obstetrics, it aids in the management of Rh incompatibility between a pregnant woman and her fetus. Furthermore, in the diagnosis of autoimmune conditions, the Coombs' test can identify the presence of autoantibodies targeting RBCs, guiding treatment decisions.

Limitations

While the Coombs' test is invaluable in many clinical scenarios, it has its limitations. False positives can occur, and the test may not detect all types of antibodies, especially those present in low titers. Additionally, the test's sensitivity and specificity can be influenced by the technique used and the individual's condition being tested.