The validity of diagnostic cut-offs for commercial and in-house scrub typhus IgM and IgG ELISAs: A review of the evidence

Abstract

Background Scrub typhus is a neglected tropical disease that causes acute febrile illness. Diagnosis is made based upon serology, or detection of the causative agent– Orientia tsutsugamushi– using PCR or in vitro isolation. The enzyme-linked immunosorbent assay (ELISA) is an objective and reproducible means of detecting IgM or IgG antibodies. However, lack of standardization in ELISA methodology, as well as in the choice of reference test with which the ELISA is compared, calls into question the validity of cut-offs used in diagnostic accuracy studies and observational studies. Methodology/Principal findings A PubMed search and manual screening of reference lists identified 46 studies that used ELISA antibody cut-offs to diagnose scrub typhus patients, 22 of which were diagnostic accuracy studies. Overall, 22 studies (47.8%) provided little to no explanation as to how the ELISA cut-off was derived, and 7 studies (15.2%) did not even state the cut-off used. Variation was seen locally in reference standards used, in terms of both the diagnostic test and cut-off titer. Furthermore, with the exception of studies using ELISAs manufactured by InBios, there was no standardization of the selection of antigenic strains. As a result, no consensus was found for determining a cut-off, ELISA methodology, or for a single value diagnostic cut-off. Conclusions/Significance We have concluded that there is a lack of consensus in the determination of a cut-off. We recommend interpreting the results from these studies with caution. Further studies will need to be performed at each geographic location to determine region-specific cut-offs, taking into consideration background antibody levels to discriminate true disease from healthy individuals.

Diagnosis of scrub typhus: recent advancements and challenges

“…Further studies will be required to determine the region-specific cut-offs to improve the accuracy of the diagnosis method. To determine the area-specific cutoff, studies need to be done in different geographical locations, and considering the background antibody levels can differentiate the diseased from healthy individuals (Saraswati et al 2019 ).…”

Selection of Diagnostic Cutoffs for Murine Typhus IgM and IgG Immunofluorescence Assay: A Systematic Review

“…As has been previously established, it is also likely that these cutoffs are not befitting for the locations in which they were being used. 41 …”

“…This should be considered when interpreting test results, as generally on infection, an increase in IgM is seen, followed by increased levels of IgG. 41 , 42 …”

Assessment of two immunoassays for detection of IgM antibodies to scrub typhus using a serum panel

“…ROC curve was constructed with OD values to discriminate the diseased from the non-diseased and the same is represented in The performance indices of ELISA at different OD cut-off values are given in Table 6. It shows that the best sensitivity and specificity and correct classification as a case of scrub typhus occurred at an OD of 1.0 (16) (serum dilution is 1:100)…”