A pseudoepidemic of Legionella infections.

During a six-month period, we observed an increase in the incidence of presumed Legionnaires' disease (LD) due to false-positive direct immunofluorescent antibody (DFA) staining. Contamination of the DFA staining reagents with Legionella appeared to account for our pseudoepidemic. Although a positive DFA stain has been regarded as highly specific for the diagnosis of LD, the clinician must interpret such results with caution.