COVID or no COVID: Interpreting inconclusive SARS-CoV-2 qPCR results in different populations and platforms

INTRODUCTION: High cycle threshold values (Ct) value) results for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) may be true infections or false-positive results. Misinterpretation of results has negative consequences. Goal of this study was to evaluate quantitative real-time polymerase chain reaction (qPCR) results with high Ct-values, to reach a point where a correct interpretation can be given.

METHODS: High Ct-value results of SARS-CoV-2 in respiratory samples taken between April 2020 and January 2021 were analysed. Three different SARS-CoV-2 qPCR assays (in-house, Alinity M and Xpert Xpress) were used for screening patients and healthcare workers (HCW). High Ct-value results were defined as "inconclusive". The Ct-value cut-off for the interpretation of the test as "positive" and "inconclusive" were based on quality assurance panel results and manufacturers' instructions.

RESULTS: Out of totally 50.295 samples tested for SARS-CoV-2, the in-house and Alinity M qPCR together yielded 379 inconclusive results. A second sample existed for 217 samples, allowing dynamics of the PCR in time. Of these, 187 were negative (86%), 11 again inconclusive (5%) and 19 positive (9%). Sixteen out of 19 persons with a positive result were HCW, 14 (74%) had a link to a SARS-CoV-2 infected person. The majority of inconclusive results detected with the Xpert Xpress (n=45 of 3603), were related to individuals with a known history of SARS-CoV-2 infection (n=28, 62%).

CONCLUSION: This study shows the importance of re-testing inconclusive SARS-CoV-2 qPCR results. Only then, the correct (true or false) interpretation can be given, leading to the right measures.