Upper limit of normal titer for Legionella pneumophila group 1 by indirect immunofluorescence and microagglutination tests in healthy population in Italy.

The background prevalence of antibody to Legionella pneumophila group 1 in two groups of healthy population of the areas of Milano and Roma, Italy, was determined by testing single serum specimens from 530 blood donors by indirect immunofluorescence (IFA) test and by microagglutination (MA). The upper limit of normal (ULN) titer was found to be 16 by IFA and 8 by MA. There was little difference in the prevalence of titers in the two geographic regions. Of the 486 subjects from whom data were available titers above the ULN value did not vary significantly with age or sex. Considering the possibility of an equivocal area IFA titers greater than 64 and MA titers greater than 16 in single serum specimens can be considered as presumptive of infection.