Determination of cut-off of diagnostic ELISA for Scrub typhus in endemic setup: Central India

Serology remains the mainstay for diagnosis of scrub typhus. In central India, diagnosis of this neglected emerging zoonotic disease suffers due to lack of standardized region-specific cutoff value and diagnostic test. Henceforth, standardized region-specific cutoff value for diagnostic ELISA play a significant role in initial treatment of patients and to differentiate other febrile diseases in endemic setup. A total of 144 patients of all age groups with acute undifferentiated febrile illness patients, forty healthy controls, dengue and chikungunya positive thirty-five samples in each category, respectively were enrolled in the study and subjected to IgM ELISA (InBioS, International, Inc.). Samples showing OD value more than 0.5 in IgM ELISA, were subjected to nested PCR. Both, receiver operating characteristic (ROC) curve and healthy volunteer samples mean with +3 SD were considered to generate region specific cutoff OD value. A total of 48 patients were diagnosed as cases of scrub typhus through IgM ELISA. Out of 48 samples, 30 were positive by nested PCR. The ROC curve analysis revealed a diagnostic ELISA cutoff value of 0.73 with sensitivity and specificity of 95% and 100%, respectively. The cut off arrived from healthy volunteer is mean OD + 3 SD is 0.72. Considering the significance of scrub typhus diagnosis for treatment and to understand disease dynamics in region wise, the cutoff value of >0.72 for diagnostic ELISA for Madhya Pradesh in central India can be used.