Coomb’s Test

Coomb's Test

Coomb’s test which is also known as Antiglobulin Test (AGT) refers to two clinical blood tests used in immunology and immunohematology which are done to find certain antibodies that cause autoimmune haemolysis of red blood cells (erythrocytes). It was discovered by Coombs, Mourant and Race in 1945 but named after discoverer Dr. Robin Coombs.

It is antiglobulin test that identifies IgG antibodies and complement protein. Coombs reagent is antihuman globulin. This test is also termed as Red blood cell Antibody screening test. It detects antibodies in bloodstream that are causing immune system to attack and destroy own red blood cells.


Presence of these antibodies indicates hemolytic anemia, a condition in which blood does not contain enough red blood cells as they are destroyed prematurely. A healthy red blood cell lives for about 120 days but in people with hemolytic anemia, red blood cells are destroyed long before 120-day marker.



In some diseased conditions, an individual’s blood may contain IgG antibodies that can specifically bind to antigens on surface membrane of red blood cell (RBC).  Red cells which are coated with complement or IgG antibodies do not agglutinate directly when centrifuged. These cells are said to be sensitized with IgG or complement.

For an agglutination to occur an extra antibody, which reacts with the Fc portion of the IgG antibody, or with the C3b or C3d component of complement, must be added to the system.  As antibodies are gamma globulins, an antibody to gamma globulin can form bridges between red cells sensitized with antibody and cause them to agglutinate.


Coombs Test


There are two types of Coomb’s test:

  1. Direct Coomb’s test
  2. Indirect Coomb’s test

Direct test detects antibodies and complement proteins that are stuck to red blood cells while indirect test looks for antibodies floating in liquid part of blood (serum).


Result interpretation

Negative Result: No clumping of cells (no agglutination). It indicates no antibodies to red blood cells.


Positive Result: Clumping (agglutination) of red blood cells. Indicates presence of antibodies on red blood cells that causes destruction of red blood cells by immune system (hemolysis). This may be due to:

  • Transfusion reaction such as one due to improperly matched units of blood.
  • Hemolytic anemia.
  • Erythroblastosis fetalis (hemolytic disease of the newborn).
  • Mycoplasma infection.
  • Chronic lymphocytic leukemia or similar disorder.
  • Systemic lupus erythematosus.
  • Infectious mononucleosis.



  1. Well established and trusted.
  2. Available in most labs.



  1. Lack of standardization.
  2. Lack of quantitation.


Download Link: Coombs Test.pdf

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