Upper limit of normal titer for detection of antibodies to Legionella pneumophila by the microagglutination test

An upper limit of normal microagglutination titer for the Knoxville strain of Legionella pneumophila was determined by testing 830 sera. The upper limit of normal titer was found to be 8; however, we suggest that when applied to single serum specimens a titer of 32 be considered the lowest titer suggestive of infection (not necessarily disease).