advantages of various testing methods
The detection methods of COVID-19 mainly include nucleic acid detection, antibody detection and antigen detection. Due to the low detection rate of antigen, the SARS-CoV-2 detection mainly focuses on antibody and nucleic acid detection. Nucleic acid detection has the characteristics of early diagnosis, high sensitivity and specificity; However, antibody detection is convenient and rapid, which can be used as a supplementary means of nucleic acid diagnosis. Due to the limitations of “false positive” and “false negative” antibody detection, it is not suitable for general population screening such as returning to work, and school, nor for epidemiological investigation in low epidemic areas.
NAT Detection process
The detection procedure needs to go through five steps: sampling, sample retention, preservation, nucleic acid extraction and computer detection.
First, collect samples according to the instructions of the kit. The sample types include pharyngeal swab, nasal swab, sputum, bronchial lavage fluid, alveolar lavage fluid, etc.
Because RNA is easy to degrade, flocked swabs without RNase and storage tubes without RNase are used when collecting samples.
After obtaining the patient’s sample, it needs to be tested as soon as possible. If it cannot be tested immediately, it needs to be packaged at low temperature and sent to a special testing institution for testing.
After receiving the sample, the testing institution shall extract the nucleic acid from the sample. The nucleic acid extraction reagent shall use the nucleic acid extraction kit specified in the approved product manual.
Finally, fluorescence PCR nucleic acid detection, that is, machine detection. It takes 70-80 minutes to amplify the extract by fluorescence PCR.
Sample collection, oropharyngeal swab or nasopharyngeal swab
NAT Detection process Nucleic Acid Testing
Antibody detection
After the virus invades the human body, the human body will produce corresponding specific antibodies for defense.
According to different molecular structures and antigen specificity, scientists divide immunoglobulins into five categories: IgG, IgA, IgM, IgD and IgE.
After the antigen enters the human body for the first time, plasma cells secrete antibodies after a certain incubation period. IgM first appeared, but the antibody had short maintenance time, low concentration and low affinity. It lasted for several days to several weeks in the blood. It was a diagnostic index of acute infection; When IgM is about to disappear, the content of IgG reaches the peak. IgG appeared late, but its concentration was high, its maintenance time was long and its affinity was high. The positive serum IgG indicates the middle and late stage of infection or previous infection. But the antibody content increases rapidly in the short term, which may be several times to dozens of times higher than the original antibody content. It is mainly IgG, which lasts a long time in the body, and IgM rarely increases.
In this epidemic, the researchers studied patients with covid-19 and found that after the virus invades the human body, IgM antibody takes about 5-7 days to produce and IgG antibody takes 10-15 days to produce.
Antibody detection kits-advantages of various testing methods
Comparison of advantages and disadvantages of two different detection methods
Compared with the serological detection of antibodies, Nucleic Acid Testing can detect patients in the window stage.
Early detection of infected persons is the “gold standard” for COVID-19 detection, but it requires higher detection equipment or platform.
The high sensitivity RT-PCR instrument is expensive, and the laboratory cleanliness and operator requirements are higher. In addition, nucleic acid detection takes a long time. Considering the sample transportation and sample backlog, the results can be reported as soon as 24 hours.
Compared with nucleic acid detection, blood samples detected by antibody serology are easier to obtain. If the RNA of the virus detected by nucleic acid is the direct evidence of the existence of the virus, the antibody detection is the antibody stimulated in the patient’s blood, which is indirect evidence and has a suggestive effect on the clinic.
When nucleic acid detection is negative, adding IgM and IgG antibody detection can make up for the shortcoming that nucleic acid detection is easy to cause missed diagnosis.
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