Coombs test
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Coombs test (also known as Coombs' test, antiglobulin test or AGT) refers to two clinical blood tests used in haematology and immunology.
The two Coombs tests:
- Direct Coombs test, also direct antiglobulin test or DAT.
- Indirect Coombs test, also indirect antiglobulin test or IAT.
Blood samples for Coombs tests are taken by phlebotomists, nurses or physicians by venipuncture. Coombs tests are done by trained laboratory staff. The clinical significance of the results are assessed by physicians.
Contents
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
- Hemolytic disease of the newborn (also known as HDN or erythroblastosis fetalis)
- *Rhesus D hemolytic disease of the newborn (also known as Rh disease)
- *ABO hemolytic disease of the newborn (the indirect Coombs test may only be weakly positive)
- *Anti-Kell hemolytic disease of the newborn
- *Rhesus c hemolytic disease of the newborn
- *Other blood group incompatibility (RhC, Rhe, RhE, Kid, Duffy, MN, P and others)
- Alloimmune haemolytic transfusion reactions
Autoimmune haemolysis
- Warm antibody autoimmune hemolytic anemia
- *Idiopathic
- *Systemic lupus erythematosus
- *Evans' syndrome (antiplatelet antibodies and haemolytic antibodies)
- Cold antibody autoimmune hemolytic anemia
- *Idiopathic cold hemagglutinin syndrome
- *Infectious mononucleosis
- *Paroxysmal cold hemoglobinuria (rare)
Drug induced immune mediated haemolysis
- Methyldopa
- Penicillin (high dose)
Rh Disease; 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 quantity 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.- Antibody screening
- Cross matching
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 1945 by Cambridge immunologists Robin Coombs (whom it is named after), Arthur Mourant and Rob Race.Coombs RRA, Mourant AE, Race RR. A new test for the detection of weak and "incomplete" Rh agglutinins. Brit J Exp Path 1945;26:255-66. Historically, it was done in test tubes. Today, it is commonly done using microarray technology and gel electrophoresis.Reference
External links
- [Coombs testing] - Institute for Transfusion Medicine.
- [Coombs’ test - direct] - Medlineplus.org.
- [Coombs’ test - indirect] - Medlineplus.org.
- [Acute Anemia] - emedicine.com
- [Drugs that cause haemolytic anemia] - Merck Manual.
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