Diagnostic Accuracy of the InBios Scrub Typhus Detect™ ELISA for the Detection of IgM Antibodies in Chittagong, Bangladesh

Abstract

Here we estimated the accuracy of the InBios Scrub Typhus Detect™ immunoglobulin M (IgM) ELISA to determine the optimal optical density (OD) cut-off values for the diagnosis of scrub typhus. Patients with undifferentiated febrile illness from Chittagong, Bangladesh, provided samples for reference testing using (i) qPCR using the Orientia spp. 47-kDa htra gene, (ii) IFA ≥1:3200 on admission, (iii) immunofluorescence assay (IFA) ≥1:3200 on admission or 4-fold rise to ≥3200, and (iv) combination of PCR and IFA positivity. For sero-epidemiological purposes (ELISA vs. IFA ≥1:3200 on admission or 4-fold rise to ≥3200), the OD cut-off for admission samples was ≥1.25, resulting in a sensitivity (Sn) of 91.5 (95% confidence interval (95% CI: 96.8⁻82.5) and a specificity (Sp) of 92.4 (95% CI: 95.0⁻89.0), while for convalescent samples the OD cut-off was ≥1.50 with Sn of 66.0 (95% CI: 78.5⁻51.7) and Sp of 96.0 (95% CI: 98.3⁻92.3). Comparisons against comparator reference tests (ELISA vs. all tests including PCR) indicated the most appropriate cut-off OD to be within the range of 0.75⁻1.25. For admission samples, the best Sn/Sp compromise was at 1.25 OD (Sn 91.5%, Sp 92.4%) and for convalescent samples at 0.75 OD (Sn 69.8%, Sp 89.5%). A relatively high (stringent) diagnostic cut-off value provides increased diagnostic accuracy with high sensitivity and specificity in the majority of cases, while lowering the cut-off runs the risk of false positivity. This study underlines the need for regional assessment of new diagnostic tests according to the level of endemicity of the disease given the high levels of residual or cross-reacting antibodies in the general population.

Keywords: Bangladesh; ELISA; diagnosis; scrub typhus.

Conflict of interest statement

The commercial assays used in this study were provided free of charge by the manufacturers and did not play a role in the design of the study or had access to the manuscript prior to submission for publication. The authors declare no personal conflicts of interest.

Figures

Figure 1

Distribution of InBios Scrub Typhus Detect™ immunoglobulin M (IgM) ELISA optical densities (ODs) compared with immunofluorescence assay (IFA) IgM for admission and convalescent samples.

Figure 2

Sensitivity (●) and specificity (■) at a range of diagnostic cut-off values for the scrub typhus IgM ELISA when compared to the reference modified scrub typhus infection criteria (mSTIC), IFA, and PCR scrub typhus diagnosis for admission (Adm-A,D,G), convalescent (F/U-B,E,H), and all samples (C,F,I).