COVID-19 test, why is it negative for a while and positive for a while?

It has been almost four months since the outbreak in COVID-19, and the epidemic in China has been brought under control, but a few patients have been positive after discharge. Why is this? Will these people infect others again?

According to the discharge standard before the sixth edition, after treatment, the body temperature returned to normal for more than 3 days, the respiratory symptoms were obviously improved, the acute exudative lesions were obviously improved according to the lung impact study, and the nucleic acid detection of respiratory specimens was negative twice in a row (the sampling time was at least 1 day). Meet the above conditions, you can release the isolation and leave the hospital. What's the problem?

One reason: there may be something wrong with the detection method itself. The test results of these patients were negative before discharge and positive after re-examination. The situation shows that the virus has not been completely eliminated. The first two negatives are not true negatives, but false negatives. The throat swab of the upper respiratory tract may not reflect the actual situation of the virus. Why is it false negative?

At present, the quantitative PCR method has high technical requirements for operators, and sample preparation or sampling is an important step that may cause false negatives.

Three genes, ORF 19-NCOV, N gene and E gene, were detected in novel coronavirus.

Take pharyngeal and tonsil secretions for bacterial culture or virus isolation. How to choose? However, over-reliance on nucleic acid detection technology, sampling, the most important step, has been largely ignored. Improper sample collection, no samples in a reasonable position were collected, and the virus content could not reflect the real situation.

Normal operation: open your mouth, expose your throat, and wipe the secretions on the two palatal arches, pharynx and tonsils with a long sterile cotton swab in the culture tube with sensitive and gentle movements. Then insert the cotton swab into the test tube and send it for inspection in time. The key step is wiping, not dipping, not lighting.

Swallow swab sampling is a very, very critical step! In other words, if the sampling is wrong, there is a high probability that the virus will not be detected, or different doctors may get the opposite result when sampling.

In the whole operation and detection process, the causes of false negatives are: 1, and improper storage and treatment of samples lead to the degradation of viral RNA during processing. 2. The extraction efficiency is low, the virus is not completely extracted, or the virus is degraded during the extraction. 3. The detection resolution is low and the detection limit is too high to detect viruses. It can be seen that both the RT-QCPR detection step and the sampling step before detection should avoid false negatives to the greatest extent.

Another possible reason is related to the cycle of virus replication. In the process of antiviral treatment and gradual improvement in the hospital, the viral load will drop to a certain extent, and it will probably not be detected. And there is a possibility that people will live with the virus, as mentioned by experts from the Ministry of Health before. COVID-19 may live with human beings for a long time. Whether the virus is active or not mainly depends on human immunity. This truth is a bit like hepatitis B virus, which turns negative after continuous medication, but it does not mean that the virus is completely eliminated.

Will these people be infected again after discharge?

Theoretically, this part has been infected by the virus, and the hospital has closed for treatment, so the viral load has decreased, and the high-tech coronavirus antibody IgG has been produced in the body. Under the normal immunity of human body, the possibility of reinfection is very small.

Are these patients still contagious? Patients who are clinically found to meet the national standards for the removal of isolation from diagnosis and treatment projects will be diagnosed by their families about 8 days after discharge. The patient's nucleic acid is positive, which can be understood as the existence of virus in the body. For the patient himself, it may not be reinfected, but it is a potential source of infection for close contacts. For epidemic control, false negative results are more terrible than false positive results, that is, a person who has been infected, but the test results are negative, can not meet the admission conditions, so letting him move freely is the biggest source of infection. For a new infectious disease, it is better to be strict.

What should I do in this situation?

First of all, the emission standards should be stricter. In the seventh edition of COVID-19's diagnosis and treatment plan just released, the discharge standard is also clearer than that in the sixth edition, that is, it was previously required to do two nucleic acid tests for respiratory tract samples, and now it is clear to do two nucleic acid tests for sputum, nasopharyngeal swab and other respiratory tract samples. This can actually reduce the false positive caused by improper sampling of nasopharyngeal swab.

Secondly, isolation should be continued after discharge 14 days. The seventh edition has also been deleted? Release the quarantine? Statement. If it is not clear whether it is still contagious, delete it from the perspective of controlling the spread of the disease. Release the quarantine? It is wise to do so. At present, Wuhan requires discharged patients to be isolated at isolation points for two weeks to reduce the risk of infection that may be caused by recurrence.

Third, try to increase the number of sampling points.

For example, in Shanghai and other cities, anal swabs need to be tested, and anal swabs (feces) can only be discharged if their nucleic acid test is negative. He said that because the discharge standard in Shanghai is stricter than the "diagnosis and treatment plan", so far, there has not been a case of nucleic acid conversion in discharged patients in Shanghai.

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