Validation of Legionella pneumophila indirect immunofluorescence assay with epidemic sera

Sera from six outbreaks of legionellosis and four outbreaks of pneumonia of other etiologies were tested with the indirect immunofluorescence assay (IFA) as currently performed. The current IFA is at least as sensitive as the original test in detecting cases of Legionnaires disease (78 to 91%). By using Center for Disease Control criteria for a positive (fourfold increase in titer during convalescence to greater than or equal to 128) or presumptive (single titer greater than or equal to 256) serological test, the specificity exceeded 99%. No cross-reactions against Legionella pneumophila antigens were observed among sera from epidemic cases of Q fever, tularemia, and psittacosis; the only positive L. pneumophila IFA titer among the epidemic Mycoplasma pneumonia sera was reduced to a negative titer with an immunosorbent extracted from Escherichia coli strain O13:K92:H4. The slight increase in specificity (to 100%), however, was offset by a slight decrease in sensitivity. The sensitivity of the IFA was maximal when a conjugate that detected immunoglobulins G, M, and A was used. IFA titers were not significantly altered by replacing the monovalent serogroup 1 antigen with a polyvalent antigen (serogroups 1 through 4) nor by the presence of rheumatoid factor or heat-labile serum factors.