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Birmingham University Turnkey laboratory is part of the Lighthouse network responsible for testing clinical samples under the UK government Test & Trace scheme. Samples are analysed for the presence of SARS-CoV-2 in respiratory samples using the Thermofisher TaqPath RT-PCR test, which is designed to co-amplify sections of three SARS-CoV-2 viral genes. Since more recent information became available regarding the presence of SARS-CoV-2 variants of concern (S-VoC), which can show a suboptimal profile in RT-PCR tests such as the ThermoFisher TaqPath used at the majority of Lighthouse laboratories, we analysed recently published data for trends and significance of the S-gene dropout variant.
Results showed that: (i) the population of S-gene dropout samples had significantly lower median Ct values of ORF and N-gene targets compared to samples where S-gene was detected (ii) on a population basis, S-gene dropout samples clustered around very low Ct values for ORF and N targets (iii) linked Ct values for individual samples showed that a low Ct for ORF and N were clearly associated with an S-dropout characteristic (iv) when conservatively inferring relative viral load from Ct values, approximately 35% of S-dropout samples had high viral loads between 10 and 10,000-fold greater than 1 x 106, compared to 10% of S-positive samples. This analysis suggests that patients whose samples exhibit the S-dropout profile in the TaqPath test are more likely to have high viral loads at the time of sampling. The relevance of this to epidemiological reports of fast spread of the SARS-CoV-2 in regions of the UK is discussed.
Preprint available in medRxiv (Dec. 27, 2020):
