Scrub Typhus Diagnosis by Enzyme Linked Immunosorbent Assay (ELISA), Indirect Immunofluorescence Assay (IFA) and Molecular Tests: Recommendations for Modification of Scrub Typhus Inclusion Criteria (STIC) in Indian Context

Enzyme linked Immunosorbent Assay (ELISA) and Immunofluorescence Assay (IFA) are specific serological tests for Scrub Typhus (ST). The etiological agent, Orientia tsutsugamushi DNA is demonstrable by PCR. Different countries apply ST IgM IFA cut-off titres ranging from 1:10 to 1:10240. To avoid confusion, Scrub Typhus Inclusion Criteria (STIC) was proposed. One criterion was a fourfold increase to or single IFA titre of ≥1:10240, later lowered to ≥1:3200. Equally sensitive and specific ELISA has no place in STIC. Modification of STIC is needed for robust laboratory confirmation of ST. One hundred and forty clinically suspected ST patients, including 58 patients who provided paired blood samples, were subjected to ST IgM ELISA, IgM IFA and Real time PCR targeting 56kDa, 47kDa and groEL genes. Among paired samples, all 58 (100%) acute sera were positive for IgM ELISA while 49 were positive in IgM IFA (84.5%). Among convalescent samples, 50 and 43 were positive in ST IgM ELISA and ST IgM IFA (86.2% and 74.1%) respectively. Regarding 82 unpaired samples, IgM ELISA and IgM IFA positivity was observed in 82 (100%) and 72 (87.8%) cases, respectively. PCR positivity in 140 ST cases for all three or any two gene targets, 56kDa, 46kDa and groEL as per STIC was 84, while an additional 40 samples were positive for any one gene. In Indian context, STIC requires modification to include IgM ELISA positivity, lower IgM cutoff IFA titre of ≥1:64 and any one gene positivity in Real time PCR.