Evaluation of clinical usefulness of the microplate agglutination test for serological diagnosis of legionella pneumonia.

Recently, a microplate agglutination test (MPAT) was established for the serological diagnosis of legionella pneumonia. To evaluate its usefulness, this study examined antibody titres in 121 serum samples serially obtained from 40 patients with pneumonia, including 17 cases of confirmed legionella pneumonia. Six of the 17 proven cases became serologically positive within 4 weeks of the onset of pneumonia as assayed by MPAT (cut-off value: four-fold rise to > or = 128 in paired sera or > or = 256 in a single serum specimen), whereas the remaining 11 cases were serologically negative despite serial examination. Four proven cases who were treated with corticosteroids in the acute phase had antibody titres <8 during the first 4 weeks of infection, after which one case showed an elevation in antibody titre for the first time, 13 weeks after the onset of disease. In contrast, all non-proven cases had antibody titres of < or = 64, and only one case developed a four-fold or greater rise in titre. These results indicate that MPAT is a useful method for the laboratory diagnosis of suspected legionella pneumonia, although several false-negative cases were observed. This suggests that the previously established MPAT criteria may require modification, possibly to slightly lower values. These data also indicate that serial examination over the first month of infection may be necessary for serodiagnosis of legionella pneumonia, especially in patients treated with corticosteroids.