Reactivity of Serum from Patients with Suspected Legionellosis Against 29 Antigens of Legionellaceae and Legionella-Like Organisms by Indirect Immunofluorescence Assay

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Abstract
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 ⩾ 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 IgO, 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 ⩾256) against single or multiple Legionella antigens occurred in 12% of 184 normal control sera. Therefore, only seroconversions to titers of ⩾128 should be considered indicative of recent Legionella infection.