Irreproducible positive results on the Cobas AmpliPrep/Cobas TaqMan HIV-1 Qual test are qualitatively different from confirmed positive results
Date
2013-10
Authors
Maritz, Jean
Journal Title
Journal ISSN
Volume Title
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Abstract
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.
Description
The original publication is available at http://onlinelibrary.wiley.com/doi/10.1002/jmv.23811/full
Keywords
PCR, Infant diagnosis, False positive, Laboratory diagnosis
Citation
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