Diagnosis of Legionella pneumophila infections by means of formolised yolk sac antigens.

Formolised yolk sac antigens of Legionella pneumophila serogroups 1-6 were used to test 1792 serum specimens from 1431 patients with respiratory illness of serological evidence of Legionnaires' disease (LD). Thirty-five patients showed titres against the serogroup 1 antigen diagnostic for LD. Only two further cases were considered to have non-serogroup I infections (both serogroup 4) indicating that such infections are rare. Titres of greater than 1/16 against the serogroup 1 antigen occur in only 3% of subjects without LD and thus the demonstration of such a titre in patients with pneumonia during the early phase of illness can alert the clinician to the likelihood of LD. The supply of serogroup 1 antigen from the Division of Microbiological Reagents and Quality Control to routine diagnostic laboratories will be continued and monovalent serogroup 2-6 antigens will continue to be made available to reference laboratories.