Impact of COVID-19 pre-test probability on positive predictive value of high cycle threshold SARS-CoV-2 real-time reverse transcription PCR test results

Gubbay, J. B.; Rilkoff, H.; Kristjanson, H. L.; Forbes, J. D.; Murti, M.; Eshaghi, A.; Broukhanski, G.; Corbeil, A.; Fittipaldi, N.; Hopkins, J.; Kristjanson, E.; Kus, J. V.; Macdonald, L.; Majury, A.; Mallo, G. V.; Mazzulli, T.; Melano, R. G.; Olsha, R.; Perusini, S. J.; Tran, V.; Allen, V. G.; Patel, S. N.

Background: Performance characteristics of SARS-CoV-2 nucleic acid detection assays are understudied within contexts of low pre-test probability, including screening asymptomatic persons without epidemiological links to confirmed cases, or asymptomatic surveillance testing. SARS-CoV-2 detection without symptoms may represent resolved infection with persistent RNA shedding, presymptomatic or asymptomatic infection, or a false positive test. This study assessed clinical specificity of SARS-CoV-2 real-time reverse transcription polymerase chain reaction (rRT-PCR) assays by retesting positive specimens from five pre-test probability groups ranging from high to low with an alternate assay. Materials and Methods: A total of 122 rRT-PCR positive specimens collected from unique patients between March and July 2020 were retested using a laboratory-developed nested RT-PCR assay targeting the RNA-dependent RNA polymerase (RdRp) gene followed by Sanger sequencing. Results: Significantly less positive results in the lowest pre-test probability group (facilities with institution-wide screening having [≤]3 positive asymptomatic cases) were reproduced with the nested RdRp gene RT-PCR assay than in all other groups combined (5/32, 15.6% vs 61/90, 68%; p <0.0001), and in each subgroup with higher pre-test probability (individual subgroup range 50.0% to 85.0%). Conclusions: A higher proportion of false-positive test results are likely with lower pre-test probability. Positive SARS-CoV-2 PCR results should be interpreted within the context of patient history, clinical setting, known exposure, and estimated community disease prevalence. Large-scale SARS-CoV-2 screening testing initiatives among low pre-test probability populations should be evaluated thoroughly prior to implementation given the risk of false positives and consequent potential for harm at the individual and population level.