Seroprevalence of Leptospirosis in a Selected Healthy Population: Determining a Cutoff Value for Microscopic Agglutination Test Titre

Abstract

Background: The Microscopic Agglutination Test (MAT) remains the serological reference test for leptospirosis serodiagnosis; however, interpretation of diagnostic titer thresholds varies widely across clinical settings. Establishing population-based upper reference limits for MAT titers is essential to distinguish background seroreactivity from clinically significant responses. Objectives: To determine the cutoff value for MAT titers in a selected healthy population and establish upper reference limits to support clinical interpretation of leptospirosis serology Methods: MAT results from 1,344 healthy pregnant women enrolled in the Rajarata Pregnancy Cohort in Sri Lanka were analysed. Each serum sample was tested against a locally optimized MAT panel comprising 11 serovars, representing 10 serogroups (Australis, Canicola, Icterohaemorrhagiae, Mini, Pyrogenes, Bataviae, Sejroe, Autumnalis, Javanica, and Semaranga). For each participant, the highest MAT titer among these pathogenic serovars was used. The 95th and 97.5th percentiles of this distribution were calculated to define the upper limit of expected background reactivity in a healthy population. Results: Of the 1,344 individuals tested, 119 (8.9%) demonstrated reactivity to at least one pathogenic serovar. The most reactive serovar was Leptospira interrogans serovar Bratislava. MAT titers ranged from 1:50 to 1:800. Specifically, 45 individuals (3.3%) had a maximum titer of 1:50, 41 (3.1%) had 1:100, 30 (2.2%) had 1:200, 2 individuals (0.14%) had 1:400 and 1:800 (0.07% each). Bootstrap analysis of the empirical distribution of MAT titers found both the 95th and 97.5th percentiles among seropositive individuals at 1/200, with a 95% CI of 1/200 to 1/400. Titers higher than 1/200 are therefore rare in this healthy population. The convergence of these empirical percentile thresholds at 1/200 supports its validity as the upper reference limit. Conclusions: A MAT titer of 1:200 defines the upper limit of background seroreactivity in this selected population of healthy pregnant women. This threshold may serve as a useful reference for interpreting leptospirosis serology in similar groups. However, its generalization to other populations should be approached with caution.