Microbiological testing
Nasopharyngeal swab or sputum samples of patients were available for testing by specific RT-PCR assays for 2019-nCoV to detect the highly conserved RdRp and variable S gene. The cycle threshold values of the sputum samples were 8–13 cycles earlier than those of throat swabs, indicating higher viral loads detected in the lower respiratory tract. It is consistent with the observations in patients with MERS who had higher viral loads in lower respiratory tract samples than in upper respiratory tract samples [22].
Serum samples for 2019-nCoV which might indicate some virus spillover from the more severely infected lung into the systemic circulation, as previously reported in patients with SARS [23]. However, the first case of 2019-nCoV in the United States report the stool and respiratory specimens from the patients tested positive by RT-PCR for 2019-nCoV, whereas the serum remained negative [24].
Importantly, the ground glass changes on chest CT scan appeared earlier than the positive for RT-PCR test in some cases. Repeat testing of nasopharyngeal swab or sputum samples are recommended in clinical suspected cases with an initially negative result.