Formalin-killed versus heat-killed Legionella pneumophila serogroup 1 antigen in the indirect immunofluorescence assay for legionellosis.

Indirect immunofluorescence assay titers of human sera obtained against the Legionella pneumophila serogroup 1 antigen (Philadelphia 1 strain) killed with 10% Formalin showed a tendency to be lower than those obtained against the reference heat-killed antigen (geometric mean titer of 194 and 370, respectively) when all other variables in the test were held constant. Test results were interpreted the same for 96% of 60 paired sera if the cutoff level used to interpret a positive test result for the formalinized antigen was lowered by one twofold dilution factor.