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 greater than or equal to 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.