Persistence of scrub typhus IgM and IgG antibodies among patients from Karnataka, India

BACKGROUND: The longevity of scrub typhus IgM and IgG are not clear. A region-specific diagnostic cut-off for enzyme-linked immunosorbent assay (ELISA) is a necessity for improved diagnosis. OBJECTIVES: To determine a region-specific optical density (OD) value cut-off for scrub typhus IgM and IgG ELISA and to study the persistence of these antibodies. METHODS: A prospective cohort study was conducted among patients diagnosed with scrub typhus admitted to Kasturba Hospital, Manipal, Karnataka, India from August 2019 to April 2023. An equal number of scrub typhus patients and healthy volunteers were enrolled to determine region-specific scrub typhus IgM and IgG ELISA OD value cut-off. A receiver operating characteristic (ROC) curve analysis was performed to determine the OD value cut-off with an optimal combination of sensitivity and specificity. The patients were followed up prospectively at varying time points up to 18 months and scrub typhus IgM and IgG ELISA were performed in all the collected samples. RESULTS: The ROC curve analysis of scrub typhus IgM revealed an optimal OD value cut-off of 1.309 with a sensitivity and specificity of 98.7% (95% CI: 93.1%-100%). The ROC curve analysis of scrub typhus IgG revealed an optimal OD value cut-off of 0.9 with a sensitivity of 71.7% (95% CI: 60.5%-81.4%) and specificity of 93.5% (95% CI: 85.7%-97.9%). At 18 months of follow-up, scrub typhus IgM (OD value > 1.309) and IgG (OD value > 0.9) were above the newly derived diagnostic cut-off in 17 (32%) and 40 (75.4%) patients, respectively. CONCLUSION: Scrub typhus IgM and IgG antibodies were persistent above the newly derived regional diagnostic cut-off for up to 18 months.