XVIII International AIDS Conference


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High positive immunoassay result confirms HIV-1 infection and avoids the need of confirmatory tests

R.J. Ochoa Jiménez1, M. Ríos Silva2

1Mexican Institute of Social Security, Colima, Mexico, 2Regional Hospital of Colima, Colima, Mexico

Background: The laboratory diagnosis of HIV-1 infection starts with testing for antibodies to HIV-1, by immunoassays (IA). According to current international guidelines, every positive HIV IA must be confirmed by supplemental testing. A high reactivity index (RI) of the HIV antibodies IA is associated with true positive result of confirmatory tests. This study was conducted to assess whether a high RI of an HIV third-generation IA is an accurate predictor of confirmatory results in patients with positive HIV antibodies test.
Methods: In a diagnostic test study, we included 393 positive anti-HIV subjects in the Colima State Laboratory of Public Health, Mexico, between February 2005 and April 2009. A third-generation enzymatic IA was used to detect HIV antibodies. The gold standard for HIV infection was a positive result of the Western Blot test (Cambridge Biotech HIV-l, Western Blot Kit, Rockville, MD) or HIV-1 RNA ≥ 2000 copies/ml (Cobas Amplicor HIV-1 Monitor 1.5, Roche Diagnostics, Branchburg, NJ).
Results: By receiver-operating characteristic analysis, the RI ≥ 11.6 was associated with highest combined specificity and sensitivity for prediction of supplemental testing, and so was defined as high positive level. There was a significant difference in the proportion of true positive results by supplemental testing between subjects with low positive IA and those with high positive IA (87.4% Vs 100%, respectively, p < 0.01). The high RI of the HIV IA showed specificity for true positive result of 100% (95% CI, 97.2-100) and likelihood ratio of 72.5.
Conclusions: A high positive HIV IA test (RI ≥ 11.6) shows positive predictive value of 100% for HIV-1 confirmatory results. The strategy based on high positive HIV IA is an acceptable alternative because can be put into practice without additional cost (most of automated analyzers generate an RI) and diminishes cost and time for HIV diagnosis.

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