Scrub typhus is an acute febrile illness caused by the obligate intracellular bacterium,Orientia tsutsugamushi. Given their affordability and ease of use, antibody based diagnostic assays can be important diagnostic tools for early detection of scrub typhus fever in resource poor countries like Nepal. Immunochromatography (ICT) and IgM ELISA are two of the routinely employed antibody based assays for diagnosis of Scrub typhus fever in Nepal, although the recommended gold standard diagnostic test is IgM Immunofluorescence assay (IFA). This study evaluated the InBios Scrub Typhus Detect™ Immunoglobulin M (IgM) ELISA and IgM Immunofluorescence assays in single serum sample at the time of admission.
Study participants (1585 suspected cases), were enrolled based on acute febrile illness with suspected scrub typhus cases in central Nepal from April 2017 to March 2018. Blood sample was collected from the suspected patients of scrub typhus, presenting with acute febrile illness. IgM antibody to Orientia tsusugamushi was detected by using Scrub Typhus Detect™ Kit (In Bios International, USA) and an in-house IgM IFA (Australian Rickettsial Reference Laboratory, Geelong, Australia. The IFA assay was performed with the Gilliam, Karp, Kato strains and O. chuto antigens following the ARRL protocol.
Statistical analysis of ELISA IgM results when compared to reference test, IgM IFA results demonstrated the following characteristics, sensitivity 84.0% (95%CI: 79.73%-87.68%), specificity 94.82% (95% CI: 93.43%-95.99%), positive likelihood ratio 16.21% (95% CI: 12.71%-20.67%), negative likelihood ratio 0.17% (95% CI: 0.13-0.21%), disease prevalence 22.08% (95% CI: 20.06%-24.21%), positive predictive value 82.12% (95% CI: 78.28-85.42%) and negative predictive value 95.44% (95% CI: 94.27%-96.38%) respectively.
The study indicated that the IgM ELISA has the sensitivity 84.0% (95% CI: 79.73%-87.68%) and specificity 94.82% (95% CI: 93.43%-95.99%). Although IgM IFA is considered the gold standard test for the diagnosis of scrub typhus cases, it is relatively expensive, requires trained personal and a microscope with fluorescence filters. Scrub typhus IgM ELISA with appropriate OD cut–off values may be the best alternative test and possible viable option for resource limited endemic countries like Nepal.