Evaluation of the Signal-to-Cutoff Ratio of the Roche Elecsys HIV Ag/Ab Test for Predicting HIV-1 Infection in a Tertiary Care Hospital in Ankara, Türkiye

Abstract

Background: False-positive results are common in HIV screening, particularly in low-prevalence settings where enzyme immunoassay-based methods are used. Accurate discrimination between true and false reactivity is essential for timely diagnosis, reducing patient anxiety, and preventing unnecessary follow-up procedures. To evaluate the diagnostic performance of the signal-to-cutoff (S/CO) ratio in predicting true HIV-1 infection in a tertiary care hospital setting using a fourth-generation HIV Ag/Ab assay. Methods: In this retrospective study, 715 reactive serum samples collected between January 2022 and December 2024 were analyzed. HIV screening was performed using a fourth-generation chemiluminescent microparticle immunoassay. Repeatedly reactive samples were subjected to confirmatory testing based on immunochromatographic principles. The optimal S/CO threshold for predicting true HIV infection was determined using receiver operating characteristic (ROC) analysis. Results: Out of 715 reactive samples, 309 (43.2%) were confirmed as true positives, while 406 (56.8%) were false positives. All false positives had S/CO values <100, and no true positives were identified among samples with S/CO <2.30. The median S/CO value for true positives was 738.0 (IQR: 239–1337), compared to 1.72 (IQR: 1.20–3.33) in false positives (p<0.001). ROC analysis identified an optimal S/CO threshold of 29.7, which yielded an area under curve of 0.997, with 97.1% sensitivity, 99.1% specificity, 91.6% positive predictive value, and 99.7% negative predictive value. The false-positive rate decreased from 56.8% at the ≥1.0 threshold to 1.96% at the ≥29.7 threshold. Conclusion: The S/CO ratio in fourth-generation HIV Ag/Ab assays demonstrates high predictive accuracy for HIV infection. An S/CO threshold of ≥29.7 effectively reduces false-positive rates while maintaining diagnostic sensitivity, providing a valuable tool for clinical decision-making prior to confirmatory testing.

Keywords

• HIV
• chemiluminescent microparticle immunoassay
• sensitivity
• specificity
• ROC analysis

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