Evaluation of the Architect HIV Ag/Ab Combo Assay in a low-prevalence setting: The role of samples with a low S/CO ratio

Background: The Architect HIV Ag/Ab Combo Assay, a fourth-generation ELISA, has proven to be highly reliable for the diagnosis of HIV infection. However, its high sensitivity may lead to false-positive results. Objectives: To evaluate the diagnostic performance of Architect in a low-prevalence population and to assess the role of the sample-to-cutoff ratio (S/CO) in reducing the frequency of false-positive results. Study design: We conducted a retrospective study of samples analyzed by Architect between January 2015 and June 2017. Positive samples were confirmed by immunoblot (RIBA) or nucleic acid amplification tests (NAATs). Different S/CO thresholds (1, 2.5, 10, 25, and 100) were analyzed to determine sensitivity, specificity, and negative and positive predictive values (NPV, PPV). ROC analysis was used to determine the optimal S/CO. Results: A total of 69,471 samples were analyzed. 709 (1.02%) were positive by Architect. Of these, 63 (8.89%) were false-positive results. Most of them (93.65%) were in samples with S/CO < 100. However, most confirmations by NAATs (12 out of 19 cases) were also recorded for these samples. The optimal S/CO was 2.5, which provided the highest area under the ROC curve (0.9998) and no false-negative results. With this S/CO, sensitivity and specificity were 100.0%, and PPV and NPV were 95.8% and 100.0%, respectively. In addition, the frequency of false-positive results decreased significantly to 4.15%. Conclusions: Although Architect generates a relatively high number of false-positive results, raising the S/CO limit too much to increase specificity can lead to false-negative results, especially in newly infected individuals.