Direct immunofluorescence in the diagnosis of Legionnaires' disease

Over a 2 1/2 year period, 61 clinical specimens from 41 patients with pneumonia of uncertain etiology were evaluated for the presence of Legionella pneumophila (serogroups 1 to 4) by immunofluorescent antibody techniques. In 13 of 19 patients with Legionnaires' disease, the diagnosis was established by fluorescent antibody (FA) staining of lung biopsies, pleural fluids, or respiratory tract secretions. In the 19 patients with Legionnaires' disease, the diagnosis was confirmed by isolation of L pneumophila by in vitro culture techniques in five or by measurement of serum antibody titers in 17. Although the FA staining technique was of limited sensitivity (68 percent), it was highly specific: no patients with non-Legionnaires' pneumonia had a false-positive fluorescent stain. In addition, the FA staining of lung tissue was positive only when performed during the first nine days of antimicrobial therapy and when an acute bronchopneumonia was noted histologically. In cases of a nonspecific interstitial pneumonitis, FA stain was always negative, and the diagnosis could be confirmed only by serum antibody measurements. Tests for serogroups 1 to 4 with a polyvalent conjugate showed that L. pneumophila serogroup 1 was the predominant strain detected in pneumonia of uncertain etiology in the Detroit area.