Abstract

We determined the optimal cutoff titers in admission and convalescent-phase samples for scrub typhus indirect immunofluores-cence assay using Bayesian latent class models. Cutoff titers of>1:3,200 in an admission sample or of a>4-fold rise to>1:3,200 in a convalescent-phase sample provided the highest accuracy (sensitivity, 81.6%; specificity, 100%). Scrub typhus is an infectious disease caused by the Gram-neg-ative intracellular bacteriumOrientia tsutsugamushi (1). Indi-rect immunofluorescence assay (IFA) is considered a serological reference test for the disease; however, the cutoff antibody titer to use remains controversial (2). Conventional cutoff titers for the IFA IgM of1:400 in an admission sample or of a4-fold rise to 1:200 in a convalescent-phase sample were first proposed by Brown et al. in 1983 (3) and have been regularly used since then (4). However, the IFA IgM has long been suspected of having a high rate of false positivity (5). In 2011, we proposed the scrub typhus infection criteria (STIC) (a combination of culture, PCR assays, and IFA IgM), in which IFA IgM cutoff titers of either 1:12,800 in an admission sample or of a4-fold rise to1:200