Postulating a positive predictive value for HIV, HBV, and HCV screening tests using electrochemiluminescent assay by comparing with ID-NAT results

Abstract

Background: In India, blood units reactive by the initial screening assays are discarded. A significant number of these are falsely positive, leading to wastage of precious blood. Also of importance is the dilemma created when it comes to donor disclosure. Hence, it is important to evaluate the positive predictive value (PPV) of the initial screening assay to identify false-positive screening results and implement a more balanced algorithm for donor disclosure and counseling. The study was undertaken with an objective of comparing signal-to-cutoff ratio (S/Co) of positive results in HBV, HCV, and HIV screening of electrochemiluminescent (ECI) assay with the results of ID-NAT so as to predict a S/Co ratio of indicative of true positives. Methods: The retrospective study was conducted in the transfusion-transmissible infection Laboratory in a Regional Blood Transfusion Centre in South India. Consecutive blood donors from January 2015 to December 2019 were evaluated simultaneously for HBV, HCV, and HIV by ECI (Vitros 3600, Ortho Clinical Diagnostics) and ID-NAT (Procleix Panther System by Grifols), and the positive results either by one or both were analyzed. Results: A total of 163,596 voluntary donor samples were evaluated. In case of HBV, a S/Co ratio of 1000 predicts a true positive more than 94% of the time. In case of HCV, a S/Co ratio of 8 predicts a true positive more than 95% of the time. Finally, a S/Co ratio of 30 predicts a true positive as well as true negative 100% of the time in case of HIV. Conclusions: It is challenging to propose a standard S/Co ratio because the performance of each assay varies across laboratories. However, the study throws light on the role of the S/Co ratio in reducing the perplexity during donor notification.