Diagnostic Accuracy of the InBios Scrub Typhus Detect Enzyme-Linked Immunoassay for the Detection of IgM Antibodies in Northern Thailand

Abstract

In this study, we examined the diagnostic accuracy of the InBios Scrub Typhus Detect IgM enzyme-linked immunosorbent assay (ELISA) and determined the optimal diagnostic optical density (OD) cutoffs for screening and diagnostic applications based on prospectively collected, characterized samples from undifferentiated febrile illness patients in northern Thailand. Direct comparisons with the serological gold standard, indirect immunofluorescence assay (IFA), revealed strong statistical correlation of ELISA OD values and IFA IgM titers. Determination of the optimal ELISA cutoff for seroepidemiology or screening purposes compared to the corresponding IFA reciprocal titer of 400 as previously described for Thailand was 0.60 OD, which corresponded to a sensitivity (Sn) of 84% and a specificity (Sp) of 98%. The diagnostic performance against the improved and more-stringent scrub typhus infection criteria (STIC), correcting for low false-positive IFA titers, resulted in an Sn of 93% and an Sp of 91% at an ELISA cutoff of 0.5 OD. This diagnostic ELISA cutoff corresponds to IFA reciprocal titers of 1,600 to 3,200, which greatly reduces the false-positive rates associated with low-positive IFA titers. These data are in congruence with the recently improved serodiagnostic positivity criteria using the Bayesian latent class modeling approach. In summary, the InBios Scrub Typhus Detect IgM ELISA is affordable and easy-to-use, with adequate diagnostic accuracy for screening and diagnostic purposes, and should be considered an improved alternative to the gold standard IFA for acute diagnosis. For broader application, regional cutoff validation and antigenic composition for consistent diagnostic accuracy should be considered.

FIG 1

Distribution and relationship between IgM ELISA ODs and IgM IFA titers. (A) Distribution of IgM ELISA ODs compared to IgM IFA titers. (B) The relationship between IgM ELISA ODs compared to IgM IFA titers with a Spearman correlation of 0.678 and a Pearson correlation r² of 0.689.

FIG 2

Overview of ELISA diagnostic cutoff ODs plotted for various diagnostic modalities. Sensitivity (A), specificity (B), and ROC area compared with ELISA diagnostic cutoff OD at 450 nm for various diagnostic modalities (C). ●, STIC; ■, IVI; ⬥, PCR; ▲, 4-fold increase IgM IFA in paired samples plus IgM IFA admission of ≥1:3,200; ▼, 4-fold increase IgM IFA in paired samples.

FIG 3

Correlation of various diagnostic modalities compared with ELISA diagnostic cutoff OD at 450 nm. Diagnostic modalities include the following: ●, STIC; ■, IVI; ♦, PCR; ▲, 4-fold increase IgM IFA in paired samples plus IgM IFA admission of ≥1:3,200; ▼, 4-fold increase IgM IFA in paired samples; ⊗, admission of ≥1:12,800.