Evaluation of the immunofluorescent antibody test for the diagnosis of human leptospirosis.

Sera from 175 patients with clinically suspected leptospirosis were tested with the immunofluorescent antibody (IFA) assay and compared with the microscopic agglutination (MA) method. Overall, the IFA test recognized all 58 MA-positive patients with leptospirosis. On acute sera testing, the IFA titer > or = 1:100 was 0.97 specific and more sensitive than the conventional MA method (sensitivities = 0.48 versus 0.17, respectively). None of the 117 MA-negative patients, 101 healthy blood donors, and 93 patients with five diseases commonly confused with leptospirosis had IFA titers > or = 1:400. However, cross-reactivity was seen with sera from patients with syphilis. On serial testing, the IFA antibody first appeared during the first week of illness, peaked by the fourth week, and generally decreased below 1:400 after the fourth month. The IFA test appears to be moderately sensitive and specific for the initial diagnosis of leptospirosis. It could replace the more complicated and less sensitive MA assay.