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Real-time reverse transcription polymerase-chain reaction (RT-PCR) is the mainstay of Covid-19 diagnosis. False-negative RT-PCR results may hamper clinical management of patients and hinder the adoption of epidemiological measures to control the pandemic. The current study was aimed at assessing whether amplification of β-glucoronidase (GUSB) gene would help estimate the accuracy of SARS-CoV-2 RT-PCR negative results in upper respiratory tract (URT) specimens. URT specimens that tested negative by SARS-CoV-2 RT-PCR displayed higher GUSB RT-PCR cycle thresholds (CT) (P=0.070) than those testing positive (median, 30.7; range, 27.0-40.0, and median 29.7; range 25.5-36.8, respectively), this reflecting poorer cellularity. Receiver operating characteristic (roc) curve analysis indicated that a CT threshold of 31.2 discriminated best between positive and negative SARS CoV-2 RT-PCRs (area under a curve, 0.66; 95% CI, 0.50-0.81; P=0.08). This cut-off yielded a true negative ratio of 89% and accuracy of 70%. The data suggested that amplification of the GUSB gene by RT-PCR may help to appraise the accuracy of negative SARS-CoV-2 RT-PCR results in patients in whom Covid-19 is eventually diagnosed.

The LightMix Modular SARS-CoV-2 (COVID-19) was used in 17 specimens. The Realquality RQ-2019-nCoV was used in six specimens. The SARS-COV-2 Real-Time PCR Kit was used in three specimens.

Raw and normalized severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA loads in 39 hospitalized coronavirus disease-19 patients with detectable levels of SARS-CoV-2 RNA and β-glucuronidase messenger RNA (mRNA) in nasopharyngeal specimens. Raw SARS-CoV-2 RNA (A) load >10 5 log 10 copies/ml, (B) load >10 4 log 10 copies/ml, (C) load >10 3 log 10 copies/ml, and (D) <10 3 log 10 copies/ml. Amplification of β-glucuronidase mRNA gene was not possible in nasopharyngeal exudate from one patient

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