Coombs test

Image:Coombs test schematic.pngThe Coombs Test (also Coombs' Test, antiglobulin test or AGT) has two forms; the direct Coombs test, also direct antiglobulin test (DAT), and the indirect Coombs test, also indirect antiglobulin test (IAT).

It is a key laboratorybloodtest used in haematologyand immunology.

It is usually performed by trained laboratory scientific staff and the clinical significance of the results are assessed by physicians.

It was first described in 1945by CambridgeimmunologistsRobin Coombs, Arthur Mourant and Rob Race. It was initially developed as a tube test method. 96-well plates and gel methods are now available.

The test uses anti-human globulin(Coombs reagent) made by injecting human globulin into animals.

Direct Coombs Test

Also known as the direct antiglobulin test (DAT).

The direct Coombs test is used to detect if an antigen-antibody reaction has taken place in-vivo and therefore establish if there is an immunological cause of haemolysiswhere it is suspected to be a cause of anaemia. The patient's red blood cells (RBCs) are washed (the patients own serum is washed away) and then incubated with anti-human globulin, which contains antibodies specific for human immunoglobulinsand complement factors. More specific Coombs reagent can be used. If human immunoglobulin or complement factors have been fixed on to the RBC surface in-vivo (ie an immune mechanism is attacking RBCs) the anti-human globulin will agglutinate the RBCs and the test is positive.

Positive direct Coombs test results are found in immune-mediated haemolysis (antibody-mediated destruction of RBCs), as occurs in -

• haemolytic disease of the newborn(HDN)
• autoimmunehaemolysis, as may be seen in systemic lupus erythematosus
• drug induced haemolysis, as may be seen from penicillins
• haemolytic transfusion reactions.

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

Indirect Coombs test

Also known as the indirect antiglobulin test (IAT). This is a two stage test.

First stage

Washed test red blood cells (RBCs) are incubated with a test serum. If antibodiesare present in the serum to antigenson the RBC surface the antibodies will bind onto the surface of the RBCs.

Second stage

The RBCs are washed and incubated with anti-human globulin. If the the test serum contains an antibody to an antigen that is present the surface of the test RBCs, the RBCs will agglutinate and the test is positive.

Examples of screening for antibodies using the indirect Coombs test

Screening blood for transfusion

The indirect Coombs test is used in in all stages of preparing bloodfor blood transfusion(in blood grouping, in screening blood for antibodies and finally in the cross match).

In blood grouping the test serum is tested against RBCs of known blood groupsand the test RBCs are tested against known serum types. In this way the blood group is confirmed from both the test RBCs and the test serum. This can be done in 96-well plates. Accuracy of all stages of blood grouping is vital.

The indirect Coombs test can be used to screen for antibodies in a test serum, by incubating the test serum with a wide range of RBCs of different antigenic types. This is used for screening for antibodies in a patients serum prior to blood transfusion. The indirect Coombs test is used in the matching of blood products for transfusion. The recipients serum is tested against the donorRBCs. This is sometimes called cross matching blood. Blood for transfusion is also screened for infections in separate processes.

Antenatal antibody screening

The indirect Coombs test is used for screening for antibodies in antenatal women. This is to detect antibodies likely to cause haemolytic disease of the newborn. Heamolytic disease of the newborn can be due to Rhesus maternal antibodies (Rh disease) or ABO maternal antibodies (ABO haemolytic disease of the newborn) and rarely to other blood group antibodies such as Kell, Kid and Duffy.

Reference

• 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.

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.

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Coombs+test Wikipedia article Coombs test.