• COVID-19 RT-PCR diagnostic assay sensitivity and SARS-CoV-2 transmission: A missing link?

      Sahahjpal, Nikhil Shri; Hon, Ephrem Chin Lip; Dallaire, Stephanie; Williams, Colin; Ananth, Sudha; Mondal, Ashis K; Rojiani, Amyn M; Hegde, Madhuri; Kolhe, Ravindra; Department of Pathology (medRxiv, 2021-03-26)
      Background The sensitivity of commercially available RT-PCR assays varies over 10,000 fold, ranging from 10 to 20,000 viral copies/ml. The reporting of high Ct value results has been under scrutiny, as the clinical significance of these values is not yet completely understood. The early detection of infected individuals (high Ct results) in the pre-symptomatic phase of the disease using highly sensitive RT-PCR methods has been argued as a strategy to prevent transmission, while on the contrary, the reporting of high Ct has been criticized as false-positive results causing unnecessary testing and having several negative implications. The purpose of this study was to verify the presence of SARS-CoV-2 genomes in samples with a wide range of RT-PCR Ct values including samples with high Ct (37 to 42) using next-generation sequencing (NGS). Methods The study evaluated a total of 547 previously positive samples tested with the PerkinElmer® New Coronavirus Nucleic Acid Detection RT-PCR kit. The samples included in this study ranged from Ct values of 17-42, with 44 samples having a Ct > 37. Of the 547 samples, 149 were sequenced using PerkinElmer NEXTFLEX Variant-Seq SARS-CoV2 assay on NovaSeq 6000, and 398 samples were sequenced using Illumina SARS-CoV-2 respiratory viral panel kits using the NextSeq 500/550 system. Results Between the two clinical laboratories, a total of ~1.95 million samples were tested using the FDAEUA PerkinElmer® New Coronavirus RT-PCR assay. Of the 1.95 million samples, ~1.72 million were negative, ~250,000 positive, and ~16,500 in the range of 37-42. Of the 547 samples sequenced, the percentage of sequencing reads that aligned to the SARS-CoV-2 Wuhan-hu-1 reference genome (NC_045512.2) ranged from 25.5% to 99.69%. All samples sequenced showed high sequence specificity to the SARS-CoV-2 virus. Low Ct samples showed complete uniform coverage across the entire 29kb SAR-CoV-2 genome. The average coverage in samples with high Ct (>37) was found to be 55.5% (range 16.1-99.2%). However, as sample Ct increased, a gradual decrease in coverage uniformity was observed for few samples. Conclusion This study demonstrates for the first time that the viral RNA is present in the high Ct value range of 37- 42 and the sequence is unique to SARS-CoV-2 confirmed using two separate sequencing assays. This confirms that the detected Ct values are reflective of the presence of the SARS-CoV2 virus and they are not an artifact or contamination. In light of the recent work highlighting the majority of transmission being pre-symptomatic/ asymptomatic, and high Ct results being observed at both the early and late phases of infection warrants further investigation into the clinical utility of high Ct results to curtail the spread of the virus.