Here is a question about nucleic acid testing. It has become an important means of pandemic prevention and control internationally. What's the purpose of large-scale testing? And in particular, what are the limitations? How will China improve its accuracy? Thank you.
Thanks for your question. The purpose of nucleic acid testing is clear; that is, early detection, reporting, isolation and treatment. After the novel coronavirus outbreak at the Xinfadi market, the local government decided to conduct a large-scale, timely nucleic acid testing effort for the market staff, as well as those who had been to the market since May 30 and those living in the 11 surrounding communities. The testing led to early detection of fresh COVID-19 cases, which played a very important role in preventing the spread of the outbreak.
Why does nucleic acid testing play such a role? The most important reason is that it has good specificity, and its positive results can be used as a "golden standard" for the diagnosis of COVID-19 infections. Of course, nucleic acid testing has its limitations with respect to its accuracy. There may be false-positive results. For example, a typical PCR amplification can turn a molecule in 100 microliters into a trillion molecules. If I pour these one trillion molecules into an Olympic-size swimming pool, there will be 400 molecules in 100 microliters of water from the pool. As long as there is one molecule, it will be amplified. Therefore, the testing is very sensitive. We can avoid the reporting of false-positive results by retesting positive specimens through strict quality control. So, there are no problems with our positive results. There is another limitation — that of false-negative results. There are more factors that may lead to the false-negative results, such as the timing of sample collection and the type of samples. Sputum samples are generally more accurate than nasopharyngeal swabs, and the latter are more accurate than oropharyngeal swabs. The method of collection also matters. Only samples that include cells with the virus can be tested positive. Besides, other limitations related to testing include the methodology and sensitivity of reagents, as well as mutation of the virus.