Legionella pneumophila serogroup 1 antibody kinetics in patients with Legionnaires' disease: implications for serological diagnosis.

To evaluate current serological criteria for Legionella pneumophila serogroup 1 (Lp1), the antibody response was prospectively studied in all patients hospitalized for Legionnaires' disease in a Swedish county during 11 y (n = 62). A 4-fold or greater rise in antibody titre to > or = 128 (the minimum convalescent antibody level for diagnosis, as recommended by the Centers for Disease Control and Prevention), using the indirect immunofluorescence antibody test, was found in 21/52 (40%) of tested patients. By referring to the titre levels in healthy residents from the local population (World Health Organization criteria), 45/52 (87%) cases were confirmed serologically. In 21 patients followed longitudinally for 10 y, the median antibody titre fell from 256 (range 32-1024) to 16 (range 2-128) in 3 y. No booster reactions were observed in any patient. After 10 y, the geometric mean titre of this clinical cohort had reached the same level as observed in the background population 5 y earlier. Titre levels in subjects exposed to Legionella from a municipal water system indicate that only 1 out of 10 of all infections are identified clinically. Indirect immunofluorescent antibody testing with local reference sera is a sensitive method for laboratory confirmation of Lp1 in an unselected pneumonia population.