False positive reactions in the indirect fluorescent antibody test for Legionnaires' disease eliminated by use of formolised yolk-sac antigen.

The indirect fluorescent antibody (FA) technique for the diagnosis of Legionnaires' disease was used to investigate an outbreak of respiratory disease in a military population. The outbreak was later shown to be caused by an adenovirus. High titres were obtained using the ether-killed antigen supplied by the Center for Disease Control (CDC), Atlanta, but not with a formolised yolk-sac antigen prepared in out laboratory. The reactivity of these sera with the CDC antigen was removed by absorption with a partly identified gram-positive bacterium, whereas sera from persons with true Legionnaires' disease were unaffected by such treatment, suggesting that such reactivity is not specific for Legionnaires' disease. The lack of reactivity with negative control sera and strong reactivity from a small group of patients in whom seroconversion was demonstrated has led us to conclude that formolised yolk-sac antigen is a reliable antigen for the diagnosis of Legionnaires' disease by the FA method. Measurement of the sensitivity and specificity of the formolised yolk-sac antigen is still in progress.