Reactivity of serum from patients with suspected legionellosis against 29 antigens of legionellaceae and Legionella-like organisms by indirect immunofluorescence assay.

Sets of sera (444) submitted for diagnostic testing for legionellosis were tested against 29 indirect immunofluorescence assay (IFA) antigens prepared from the characterized Legionella species and Legionella-like organisms to determine the prevalence of antibodies to Legionella organisms. Reciprocal titers of 15% of the serum sets rose fourfold or more to greater than or equal to 128 (indicating seroconversion) against one or more Legionella antigens. The specificity of the test was 96% when evaluated in patients with pneumonia due to non-Legionella organisms. Antibodies were of the IgG, IgM, and (infrequently) IgA classes and were either specific for a single species (as defined by a difference in titer of fourfold or more) or reacted with common Legionella antigens (30 [45%] vs. 36 [55%] of 66 seroconversions, respectively). No single antigen detected half of the positive sera. Elevated IFA titers (of greater than or equal to 256) against single or multiple Legionella antigens occurred in 12% of 184 normal control sera. Therefore, only seroconversions to titers of greater than or equal to 128 should be considered indicative of recent Legionella infection.