Abstract
In ELISAs, sera of individuals infected by Trypanosoma cruzi show absorbance values above a cut-off value. The cut-off is generally computed by means of formulas that need absorbance readings of negative (and sometimes positive) controls, which are included in the titer plates amongst the unknown samples. When no controls are available, other techniques should be employed such as change-point analysis. The method was applied to Bolivian dog sera processed by ELISA to diagnose T. cruzi infection. In each titer plate, the change-point analysis estimated a step point which correctly discriminated among known positive and known negative sera, unlike some of the six usual cut-off formulas tested. To analyse the ELISAs results, the change-point method was as good as the usual cut-off formula of the form “mean + 3 standard deviation of negative controls”. Change-point analysis is therefore an efficient alternative method to analyse ELISA absorbance values when no controls are available.
Highlights
In continuous diagnostic clinical tests, the establishment of a reliable cut-off is of paramount importance to discriminate between infected and non-infected individuals
The scope of the present study is to evaluate the change-point analysis as a tool to identify positive Enzyme-linked immunosorbent assay (ELISA) reactions when no controls are available
All other formulas failed in some instances, giving in some plates cut-off values sometimes below the highest known negative control values (F3 or F4), or above the lowest positive control values (F2) (Table II)
Summary
In continuous diagnostic clinical tests, the establishment of a reliable cut-off is of paramount importance to discriminate between infected and non-infected individuals. Several standard methods have been proposed to choose optimal cut-offs (Lopez-Raton et al 2014), and all require known positive and negative individuals to compute the cut-off value that will best discriminate. Enzyme-linked immunosorbent assay (ELISA) is a diagnostic tool carried out commonly in parasitological studies to detect antibodies or antigens related to a specific parasite. They produce absorbance readings, and to discriminate amongst positive and negative results, a cut-off value is needed. To detect infection by Trypanosoma cruzi, the causative agent of Chagas disease, Pan et al (1992) have proposed another formula that takes into account negative and positive controls: Cutoff = Xneg + 0.13 Xpos (2). Where Xneg is the mean of the negative controls, and Xpos the mean of the positive controls
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