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Coombs test

Named for British physician Robert Roystom Amos Cooms (1912-2006)
Synonym: antiglobulin test, AGT
German: Coombs-Test

1 Definition

A Coombs test is a diagnostic procedure to detect antibodies against erythrocytes. It is performed if there is suspicion of hemolysis caused by antibodies (e.g. autoimmune hemolytic anemia, AIHA or Rh incompatibility).

2 Test procedure

A Coombs test detects antibodies that independently cannot effect the agglutination of red blood cells. Those antibodies belong to the group of IgG antibodies. They are also referred to as "incomplete antibodies". IgM antibodies can cause agglutination without reaction-enhancing substances due to their pentameric structure, which is why they can be referred to as "complete antibodies".

In order to determine antibodies, the Coombs test uses so-called Coombs serum or anti-human globulin (AHG). Coombs serum is extracted from rabbit serum which has previously been immunized against human IgG antibodies.

The test can be performed in a test tube or using column agglutination techniques.

3 Types of tests

The Coombs test can be performed directly or indirectly:

3.1 Direct Coombs test

The direct Coombs test determines IgG antibodies that bind to red blood cells. The first step is to clean the erythrocytes from the patient's blood plasma and subsequently incubate them together with Coombs serum (antibodies against human IgG antibodies and complement components). If the patient's blood contains antibodies against red blood cells and they are bound to the erythrocyte, the antibodies in the Coombs serum will bind to the human IgG which results in agglutination; a positive direct Coombs test.

3.2 Indirect Coombs test

The indirect Coombs test requires two steps. It detects antibodies against foreign red blood cells that freely circulate in the patient's serum, meaning that they are not bound to the patient's red blood cells.

The first step is to incubate the serum with specific reagent red blood cells. If the patient's serum contains antibodies, they will bind to the reagent red blood cells, although agglutination will not occur.

The second step requires adding Coombs serum to the reagent red blood cells that have previously been treated with the patient's serum. If agglutination occurs now, the indirect Coombs test is positive.

The indirect Coombs test is used in case of Rh incompatibility to detect incomplete antibodies in the mother's serum.

4 Use

The term Coombs test strictly speaking only describes the analysis technique, meaning the use of anti-human globulin.

The indirect Coombs test is used for the process of cross-matching before a blood transfusion or to detect specific antibodies but it can also play a role in determining antibody specificity.

Because of those different uses, the term Coombs test is not an exact parameter but rather an analysis technique that can be used to determine different parameters. It most commonly refers to the parameter "antibodies against erythrocyte antigens in the patient's serum".

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