Comparison of Diagnostic Performance of Commercially Available Serological and Molecular Tests for Detection of Orientia tsutsugamushi in South Korea: A Single-Center Prospective Study

<b>Background</b>: Scrub typhus is commonly misdiagnosed because of nonspecific clinical features and limited data on the performance of diagnostic tests. This study aimed to evaluate the accuracy of commercially available serological and molecular assays for diagnosing scrub typhus. <b>Methods</b>: Adult patients with suspected scrub typhus who visited a tertiary-care hospital in South Korea from July 2022 to December 2024 were prospectively enrolled. Scrub typhus was confirmed by either a positive real-time polymerase chain reaction (PCR) result for <i>Orientia tsutsugamushi</i> or a ≥ four-fold increase in the <i>O. tsutsugamushi</i>-specific total immunoglobulin (Ig) antibody titer on an immunofluorescence assay (IFA). The diagnostic performances of the serial IFA, an immunochromatography-based rapid diagnostic test (ICT), and multiplex real-time PCR targeting the <i>groEL</i> and <i>47-kDa</i> genes were compared. <b>Results</b>: Among 159 patients, 81 had scrub typhus and 78 did not. The sensitivity and specificity were 64% and 100% for the serial IFA, 75% and 91% for the ICT, and 95% and 100% for multiplex PCR, respectively. The area under the curve was significantly higher for the ICT (0.819) than for the acute-phase IFA (0.743, <i>p</i> = 0.02). <b>Conclusions</b>: Multiplex real-time PCR provided rapid and highly accurate confirmation of scrub typhus, and an acute-phase ICT may be an alternative to a single acute-phase IFA for early clinical decision-making.