Let’s talk about the coronavirus test

      Why should re sampling be conducted for recheck if the initial screening is positive?

      The covid-19 nucleic acid test must go through two processes: the first is the first test (primary screening) of the testing institution. If the result is positive, the person will be considered as the primary screening positive person. Step 2: all positive samples of primary screening need to be rechecked by higher qualified institutions to ensure that each result is accurate.

      After resampling and repeated testing, it may be determined that the coronavirus nucleic acid is positive, or the retest result may be negative, “suspicious samples” are excluded. Generally, there are two possibilities for suspicious samples: one is that CT is located in the gray area, and the other is that a single target gene of the virus appears positive due to some unexpected circumstances.

     The purpose of initiating emergency response in the event of a first screening positive person is to make full use of the golden 4-hour emergency response time after receiving the report, to prevent and control the whole chain accurately and quickly, and to control the epidemic in the bud as much as possible.

     Why do some people have positive nasal swabs? Pharyngeal swab negative? Even a anal swab?

     Since novel coronavirus is transmitted through the respiratory tract, it has different clinical significance to collect samples from different parts. In addition to the well-known nasal swabs and pharyngeal swabs, sputum, blood and anal swabs can also be collected:

Nasal swabs and pharyngeal swabs – upper respiratory tract specimens

Sputum – lower respiratory tract specimen

Anal swab – alimentary tract specimen

Blood – antibody test specimen

         The viral load of upper respiratory tract specimens (nasopharyngeal swabs and oropharyngeal swabs) is high in the first 1-3 days of clinical symptoms, and begins to slowly decrease after a week. The viral load of lower respiratory tract specimens (sputum) usually reaches a peak within 2 weeks after the onset of the disease, and the viral load is higher than that of upper respiratory tract specimens. The viral load of stool samples usually reaches a peak in 2-3 weeks after the onset of the disease, and the overall viral load is lower than that of sputum samples, The antibody in the serum can be detected after one week, and it can also determine whether there has been infection in the past.

       Therefore, in the recent cases, it is mentioned that the patient has infection symptoms, but the respiratory tract specimen is negative for coronavirus nucleic acid, multiple collection can be carried out at multiple points and locations.

       For example, the New England Journal of Medicine recently published an article describing the relationship between the viral load of nasal swabs and pharyngeal swabs and the disease progression, revealing the dynamic changes of the viral load of novel coronavirus after the onset of symptoms. In the early stage of COVID-19, the viral load was high and then gradually decreased.

       Why is there an acid positive condition in the antigen negative nucleus?

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        A review on lancet in February 2022 indicated that antigen detection and RNA detection occur within 0-7 days of infection. The sensitivity of antigen detection is far lower than that of RNA detection. Antigen detection can only detect 105-106 copies, while RNA detection can reach 102-103 copies. Antibody detection usually occurs 7-14 days after infection and includes IgG and IgM.

       It can be seen that when the viral load is small, the antigen cannot be detected, but it can be detected by high-sensitivity RNA detection. This is the reason why the antigen is positive. It also explains why the CDC recommends that antigen and RNA detection complement each other.

    How long will there be a positive to negative change? 

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       The influencing factors of negative conversion are complex, including the autoimmune system, whether there are basic diseases, and the vaccine situation. Among them, the immune system is like the bodyguard of the human body. When the immunity is strong, the virus is weak, and it may recover quickly. Maybe after a sleep, it will turn positive to negative; When the immunity is weak, the virus is strong and needs a period of time to recover.

      It is precisely because of the complex tug of war between the virus and the immune system that regular intensive sampling and detection are needed to monitor the progress of the disease in real time.