COLD AGGLUTINATION TEST

Principle: 
Cold agglutination test is a heterophile tube agglutination test. Patients suffering from primary atypical pneumonia due to Mycoplasma pneumoniae produce antibodies that react and agglutinate suspension of human O group RBC. Autoantibodies that agglutinate human erythrocytes at temperature less than 37^oC are called cold agglutinins. These are IgM antibodies to the I antigen of human RBC. They appear during the first week of primary atypical pneumonia due to Mycoplasma pneumoniae and disappear by 6 weeks in most cases. These autoantibodies are not unique to infection with Mycoplasma pneumoniae but are found in several other diseases including infectious mononucleosis, rubella, influenza, trypanosomiasis, blood dyscrasias, liver disease and hemolytic anemia. 

Procedure: 
Blood sample should be kept at 37^oC until serum is removed from the clot. Serial dilutions of patient serum are mixed with 5% suspension of human O group RBC, incubated at 4^oC for one hour and read for haemagglutination. This haemagglutination is reversed if the tube is incubated at 37^oC for 15-30 minutes.

Interpretation: 
A convalescent titre of >32 or a four-fold rise in titre between acute and convalescent sera is considered significant. Because only half of patients with Mycoplasma infection produce cold agglutinins, the test is only a screening test. Besides, the test is positive in several other cases making it a non-specific test.