International and regional standardisation of immuno-enzyme tests: methods, advantages and limitations

Numerous attempts have been made to standardise immune-enzyme techniques (enzyme-linked immunosorbent assay: ELISA) used for the diagnosis of infectious diseases, in order to improve the reproducibility of the tests, expression of results, choice of a positive threshold, and selection of reference samples. The international standardisation of reagents and test protocols appears essential for quality central and the comparison of results between laboratories, but the interpretation of results can encounter major differences depending on the geographical sector under study. Based on these studies, and in the light of a model indirect ELISA for detecting antibodies against Trypanosoma vivax in cattle, the author proposes the international standardisation of reagents, test protocol, and the expression of results of ELISA using international reference samples. For local standardisation, the following proposals are made: - sampling of representative local populations - establishment of the distribution patterns of infected and uninfected local populations - selection of representative controls from local populations (secondary reference samples) - expression of test results in comparison with these controls - establishment of internal quality control based on the response of controls - determination of a positive threshold, in accordance with the requirements of the user - adaptation of the positive threshold according to the prevalence observed in the geographical sector under study. These measures will make it possible to determine sensitivity and specificity of the test in the population studied and, when the prevalence of infection is known, to calculate the predictive values of the test.