Onset and duration of urinary antigen excretion in Legionnaires disease.

The purposes of this study were to determine whether antigen is excreted by patients with Legionnaires disease early enough after the onset of symptoms to be useful for making therapeutic decisions and whether antigen excretion ends when successful treatment is concluded. Specific antigen was detected in the urine of 14 (88%) of 16 patients with Legionnaires disease during days 1 to 3 of symptoms, 33 (80%) of 41 patients during days 4 to 7, 25 (89%) of 28 patients during days 8 to 14, and 11 of 11 patients after day 14, by solid-phase immunoassays for serogroup 1 Legionella pneumophila antigen. Antigen excretion persisted for 42 days or longer after the onset of treatment in at least 15 patients. The longest documented duration of excretion was 326 days. We conclude that antigen can be detected approximately as often early after symptoms begin as later, allowing meaningful therapeutic decisions to be made, but that prolonged antigen excretion may negate the diagnostic value of urinary antigen detection for relapsing or recurrent L. pneumophila pneumonia.