Validation of a New Scrub typhus Rapid Kit-Standard Q Tsutsugamushi IgM/IgG

A new rapid kit manufactured in India, Standard Q Tsutsugamushi IgM/IgG (SD Biosensor) for Scrub typhus diagnosis targeting both IgM and IgG antibodies to Orientia tsutsugamushi, is validated for its performance. The result obtained is compared keeping ST IgM and ST IgG ELISA (Scrub Typhus Detect IgM/IgG ELISA System, InBios International, California, USA) as reference standard. Percentage sensitivity, specificity, positive predictive value and negative predictive values for SD Biosensor IgM were 97.52, 96.90, 96.72 and 97.66 respectively, with a kappa value of 0.944, which is considered as 'very good'. However with reference IgG antibody detection, the percentage sensitivity, specificity, positive predictive value and negative predictive value were 49.15, 98.48, 47.20 and 68.42 respectively with a kappa value of 0.489, which is considered as 'moderate'. In acute scrub typhus, IgM antibodies predominate and are consistently present during the early days of illness. Presence of ST IgG antibodies points to either a past infection or relapse/recurrence. This kit may be useful for diagnosing acute scrub typhus for IgM antibodies because of its cost-effectiveness. Inclusion of strong positive and week positive controls in the kit is recommended so as to compare with the test results. As is the case with all rapid kits, regular and frequent monitoring of the performance is mandatory and clinical correlation is essential.