2. Nucleic acid detection

It mainly refers to the polymerase chain reaction, which is commonly referred to as PCR detection. The nucleic acid can be detected as long as it infects the mucosa), and the relatively closed operating environment can reduce the pollution of the sample to the environment.

However, its disadvantage is that it has high special requirements for laboratory hardware equipment and personnel; the overall detection time is long, and a round of detection often takes more than 2-3 hours; the throughput of most PCR instruments is limited to 96 wells, including internal Targeted use, the actual patient sample testing volume is less. At the same time, the problem of PCR is that it is very easy to contaminate, and a small amount of contamination can cause false positives.

Since the outbreak of the epidemic, there are incomplete statistics suggesting that "nucleic acid testing has a high positive rate of 30-50% for positive patients. That is to say, some patients have negative nucleic acid test results, but are clinically confirmed as suspected new crowns. Pneumonia is a false negative. There are even reports that individual patients have been tested negative for seven nucleic acid tests, and it was not until the eighth time that a positive result was detected. Therefore, some clinicians and the public began to question the inability of this test to work. On this issue, we also The reasons are analyzed:

1. Characteristics of 2019-nCoV virus: The industry's awareness of the new coronavirus is still gradually increasing. It is not clear whether the content and distribution of the virus in body fluids and the severity of symptoms are linearly positively correlated after patients are infected with the virus. If there are clinical symptoms, but the virus has not been rapidly replicated and released in the lungs, sampling at this time may not be able to diagnose due to the insufficient amount of virus collected. Especially in the early stage of the disease, there may be no cough, and the amount of virus in the upper respiratory tract, including the nasopharynx, is very small, and the test result is likely to be negative. However, such patients are usually less contagious, because the upper respiratory tract is not detected, suggesting that it is not highly contagious.

2. The impact of the pre-test stage: Due to the limitations of operability, the most common sampling methods at present are to use nasopharyngeal swabs, sputum or bronchoalveolar lavage fluid collection, of which nasopharyngeal swabs are commonly used. The collection of nasopharyngeal swabs has inherent limitations and is highly random; secondly, the kit requires virus inactivation, and the new crown, as an RNA virus, has low stability. The efficiency of nucleic acid extraction after sampling will be affected to a certain extent by individual differences, sampling operations and the degree of experimental operation specifications.

In clinical testing, sampling→submission for inspection→inactivation→extraction→detection, any problems in any of these steps may lead to the inability to extract sufficient and effective viral nucleic acid, resulting in false negative results in subsequent tests. It is recommended that qualified medical institutions choose deep sputum samples, and bronchoalveolar lavage fluid for severe tracheal intubation patients.

3. Interpretation of test results: Some reagent instructions describe that only two targets must be positive at the same time to report positive, but the new crown is not stable as an RNA virus and is prone to mutation. Is it also possible to report the result of only one target as negative? One of the causes of false negatives deserves further study. In addition, the correct judgment of some results requires an experienced experimenter to adjust the response curve.