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Highly sensitivity antigen assays in confirmation of HIV and HCV Ag-Ab Combo tests reactivity: possible rationalization of confirmation testing

Introduction. As blood donors in Croatia are tested for HIV and HCV antigens (Ag) and antibodies (Ab) mostly by combined Ag-Ab assays, differentiation between Ag and Ab is required on repeat reactivity (RR) confirmation. At our laboratory, confirmation is performed by use of alternative tests and compulsory confirmatory immunoblot (IB) assay. In addition, indeterminate and confirmed positive donors undergo molecular testing. In recent years, two antigen assays have been introduced: Vidas HIV DUO ULTRA & HIV p24 Ag and Architect HCV Ag test. These assays are superior to Ag-Ab combination assays in terms of sensitivity against Ag, offering a higher level of certainty in the evaluation of blood donor reactivity and final status against future donation: Aim. The idea was to introduce primary confirmatory testing to complete confirmation based on negative results yielded by one, two or more assays, in order to reduce the number of IB assays performed. This attempt was stimulated by a great number of samples referred for confirmation mostly due to HCV reactivity. Material and methods. A total of 289 HIV and 585 HCV RR blood donor samples were tested by a number of assays, as follows: Prism anti HCV, Monolisa HCV Ag-Ab ULTRA, Murex HCV Ag-Ab Combination, Architect HCV Ag, and RIBA HCV 3.0 and/or INNO-LIA HCV Score IB assays for HCV ; and Prism HIV Ag-Ab, Genscreen HIV Ag-Ab ULTRA, Vidas HIV DUO ULTRA, and RIBA HIV-1/HIV-2 and/or INNO-LIA HIVI/II Score IB assays for HIV. According to HIV and HCV confirmed status, test results showed 6 and 21 positive, 7 and 45 indeterminate, and 276 and 519 negative samples, respectively. Specificity, sensitivity, negative predictive value (NPV), positive predictive value (PPV) and likelihood ratio (LR) for positive and negative results were determined for the assays employed in order to identify the best test combination for primary confirmatory testing. Results. Result analysis was based on the following hypotheses: 1) the assays used on RR determination varied according to antigen composition, which greatly influences IB assay results and their interpretation ; 2) a positive antigen finding is a sign of infection ; 3) a finding of HIV antibody is also indicative of infection irrespective of the presence of p24 antigen ; and 4) a finding of HCV antibody without HCV antigen is not indicative of infection. Therefore, HIV IB indeterminate results were marked positive and HCV indeterminate results negative on statistical processing. Conclusion. The results presented in the table show that primary confirmatory testing could be introduced for HCV with any combination of assays, preferably Architect HCV Ag and Prism anti HCV (LR+ 5.96:LR- 0.05). However, the LR values obtained for HIV had only minor contribution on the likelihood of false reactivity confirmation in primary testing (LR- >0.48) and should not be introduced in routine. In addition, the interdependence of test type of initial testing and test combination in primary testing for HCV should be reassessed.

HIV and HCV blood donors testing; confirmation algorithms for HCV and HIV

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