Microagglutination test for early and specific serodiagnosis of tularemia

A microagglutination test with safranin-stained Francisella tularensis antigen was compared with a conventional tube agglutination test for the serodiagnosis of tularemia. The microagglutination test was performed in round-bottom microtiter plates by using 0.025 ml of the antisera and of the antigen. The antibody titers obtained by using the microagglutination test were 8 to 64 times higher than those seen with the tube agglutination. By the microagglutination test, the serum agglutinins were detected 3 days earlier in rabbits and 9 days earlier in humans than by the tube agglutination test. The microagglutination test also detected residual circulating antibodies in humans more than 20 years after recovery from infection. These early agglutinins were shown to be in the immunoglobulin M class because of their sensitivities to 2-mercaptoethanol. No significant group agglutination reaction with Brucella abortus was observed. These observations indicate that the microagglutination test is a useful tool for the early and specific serodiagnosis of tularemia.