Irreproducible positive results on the Cobas AmpliPrep/Cobas TaqMan HIV-1 Qual test are qualitatively different from confirmed positive results

Maritz, Jean
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Criteria that define low positive results on the COBAS® AmpliPrep/COBAS® TaqMan (CAP/CTM) HIV- 1 Qual test as inconclusive have been adopted by all academic centres in South Africa that conduct infant HIV PCR, following previous investigations that showed poor specificity of these results. Retesting all these specimens has considerable cost implications. It was therefore attempted to better characterise such inconclusive results, by comparing those that prove to be either negative or positive on follow-up testing. This retrospective, laboratory-based study found that 193 of 211 (91.5%) patients with with previous inconclusive results (defined as reported positive by CAP/CTM but with cycle threshold [Ct] values of >32 and/or fluorescence intensity [FI] values of <5) tested negative and only 18 (8.5%) positive using independently obtained follow-up samples after a median of 28 days). The only significant independent predictor of a later positive result was a higher FI value (3.326 vs. 0.495, p<0.0001), whereas Ct values were not independently predictive. Specimens from patients negative on follow-up testing were qualitatively different from specimens that proved to be true positive. As the lower FI values in false-positive compared to true-positive results are probably indicative of a non-specific signal, the incorporation of stringent amplification slope criteria in the assay’s test definition file may improve correct classification and thus reduce the need for repeat testing of a large number of inconclusive specimens.
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PCR, Infant diagnosis, False positive, Laboratory diagnosis
Maritz, J., Van Zyl, G. U. and Preiser, W. 2013. Irreproducible positive results on the Cobas AmpliPrep/Cobas TaqMan HIV-1 Qual test are different qualitatively from confirmed positive results, Journal of Medical Virology, 86(1):82-87, doi: 10.1002/jmv.23811