Prevalence of antibody to Legionella pneumophila in middle-aged and elderly Americans.

An indirect fluorescent antibody (IFA) test was used to establish the background prevalence of antibody to Legionella pneumophila in single serum specimens from 1,143 persons. The serum specimens had been obtained from volunteers 46 years of age and older who were not acutely ill and who resided in the areas of Atlanta, Georgia; Washington, D.C.; Houston, Texas; and Rochester, New York. The overall prevalence of seropositivity (reciprocal titer, greater than or equal to 64) was 1.7%. The prevalence of seropositivity did not vary with age, sex, or geographic location. Groups of persons in which the prevalence of reciprocal titers of greater than or equal to 64 is significantly higher than 1.7% may have unusually great exposure to L. pneumophila. In the population tested, a reciprocal IFA titer of greater than or equal to 64 would have a specificity of 98.3% in the diagnosis of an acute illness as Legonnnaires' disease.