There is an urgent need for the development of serodiagnostic approaches with improved sensitivity for patients with acute
leptospirosis. Immunoblots were performed on 188 sera collected from 74 patients with laboratory-confirmed early leptospiral
infection to detect
immunoglobulin M (
IgM)
antibodies to
antigens pooled from 10 leptospiral strains prevalent in Thailand. Sera from patients with other febrile diseases served as controls.
IgM reactivity to seven distinct
antigens, with apparent molecular masses of 14 to 18, 19 to 23, 24 to 30, 32, 35/36, 37, and 41/42 kDa, was observed. The low-molecular-mass 14- to 18-kDa band was the most frequently detected
antigen, being recognized in sera from 82.4% of patients during the first 3 days after the onset of symptoms. We evaluated the accuracy of the
IgM immunoblot (
IgM-IB) test by using reactivity to the 14- to 18-kDa band and/or at least two bands among the 19- to 23-, 24- to 30-, 32-, 35/36-, 37-, and 41/42-kDa
antigens as the diagnostic criterion. The sensitivities of the
IgM-IB test and the microscopic agglutination test (MAT) were 88.2% and 2.0%, respectively, with sera from patients 1 to 3 days after the onset of symptoms. In contrast, the
IgM-IB test was positive with only 2/48 (4.2%) sera from patients with other febrile illnesses. The high sensitivity and specificity of the
IgM-IB test for acute
leptospirosis would provide greatly improved diagnostic accuracy for identification of patients who would benefit from early
antibiotic intervention. In addition, the
antigens identified by the
IgM-IB test may serve as components of a rapid, accurate, point-of-care diagnostic test for early
leptospirosis.