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The two Coombs tests:
''Coombs test'' can refer to either the ''direct'' or ''indirect'' form. The form of the Coombs test may not be specified but it is usually clear from the clinical context or why the test was requested by a Physician that the direct or indirect form is referred to. Blood samples for Coombs tests are taken by Phlebotomist s, Nurse s or physicians by Venipuncture . Coombs tests are done by trained laboratory staff. The clinical significance of the results are assessed by Physician s. DIRECT COOMBS TEST The direct Coombs test (also known as the direct antiglobulin test or DAT) is used to detect if antibodies or complement system factors have bound to RBC surface antigens '' In Vivo ''. Examples of diseases that give a positive direct Coombs test The direct Coombs test is used clinically when immune mediated Haemolytic Anaemia (antibody-mediated destruction of RBCs) is suspected. A positive Coombs test indicates that an immune mechanism is attacking the patients own RBCs. This mechanism could be Autoimmunity , Alloimmunity or a drug induced immune mediated mechanism. Alloimmune haemolysis
Autoimmune haemolysis
Drug induced immune mediated haemolysis
(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 '''D'''isease''; ''R'' = RBCs, '''''D''''' = DAT.) Laboratory method The patient's Red Blood Cells (RBCs) are washed (the patients own Serum is washed away) and then incubated with Antihuman Globulin (also known Coombs reagent). If immunoglobulin or complement factors have been fixed on to the RBC surface In-vivo , the antihuman globulin will Agglutinate the RBCs and the direct Coombs test will be positive. (A visual representation of a positive direct Coombs test is shown in the upper half of the schematic). INDIRECT COOMBS TEST The indirect Coombs test (also known as the indirect antiglobulin test or IAT) can detect very low concentrations of antibodies. It is used to screen for antibodies in blood transfusion preparation and it is also used in antenatal care to screen pregnant women for antibodies that may cause Hemolytic Disease Of The Newborn . By diluting a serum containing antibodies the quantitiy of the antibody in the serum can be gauged. This is done by using doubling dilutions of the serum and finding what is the maximum dilution of test serum that is able to produce agglutination of relevant RBCs. Examples of clinical uses of the indirect Coombs test Blood transfusion preparation The indirect Coombs test is used to screen for antibodies in the preparation of Blood for Blood Transfusion . Donor blood and recipients blood must be ABO and Rhesus D compatible. Donor blood for transfusion is also screened for infections in separate processes. Accuracy in all the stages is vital.
A blood sample from the recipient and a blood sample from every unit of Donor blood is screened for antibodies with the indirect Coombs test. Each sample is incubated against a wide range specialy selected RBCs that together exhibit a full range of surface antigens (ie Blood Types ).
The indirect Coombs test is used to test a sample of the recipients serum against a sample of the Blood Donor's RBCs. This is sometimes called cross matching blood. Antenatal antibody screening The indirect Coombs test is used to screen antenatal women for IgG Antibodies , that are likely to pass through the Placenta into the foetal blood and cause Haemolytic Disease Of The Newborn . Laboratory method 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). First stage Washed test Red Blood Cells (RBCs) are incubated with a test serum. If the serum contains Antibodies to Antigens on the RBC surface, the antibodies will bind onto the surface of the RBCs. Second stage The RBCs are washed three or four times with isotonic saline 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. 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 Globulin . It is made by injecting human globulin into animals. Coombs reagent contains animal antibodies specific for human Immunoglobulins and human Complement System factors. More specific Coombs reagent or Monoclonal Antibodies can be used. HISTORY OF THE COOMBS TEST The Coombs test was first described in s. Today, it is commonly done using Microarray technology and Gel Electrophoresis . REFERENCE EXTERNAL LINKS
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