Comparison of qualitative and quantitative analyses of COVID-19 clinical samples

Qualitative and quantitative detection of nucleic acids of SARS-CoV-2, the pathogen that causes coronavirus disease 2019 (COVID-19), plays a significant role in COVID-19 diagnosis, surveillance, prevention, and control.

A total of 117 samples from 30 patients with confirmed COVID-19 and 61 patients without COVID-19 were collected. Reverse transcriptase quantitative PCR (RT-qPCR) and droplet digital PCR (ddPCR) were used for qualitative and quantitative analyses of these samples to evaluate the diagnostic performance and applicability of the two methods.

The positive detection rates of RT-qPCR and ddPCR were 93.3% and 100%, respectively. Among the 117 samples, 6 samples were tested single-gene positive by RT-qPCR but positive by ddPCR, and 3 samples were tested negative by RT-qPCR but positive by ddPCR. The viral load of samples with inconsistent results were relatively low (3.1-20.5 copies/test). There were 17 samples (37%) with a viral load below 20 copies/test among the 46 positive samples, and only 9 of them were successfully detected by RT-qPCR. A severe patient was dynamically monitored. All 6 samples from this patient were tested negative by RT-qPCR, but 4 samples were tested positive by ddPCR with a low viral load.

Qualitative analysis of COVID-19 samples can meet the needs of clinical screening and diagnosis, while quantitative analysis provides more information to the research community. Although both ddPCR and RT-qPCR can provide qualitative and quantitative results, ddPCR showed higher sensitivity and lower limit of detection than RT-qPCR, and it does not rely on the standard curve to quantify viral load. Therefore, ddPCR offers greater advantages than RT-qPCR.