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How soon can the nucleic acid test come out?
The test results of nucleic acid in throat swab can be obtained in 6 hours at the earliest, but due to the large amount of detection at present, the examinee usually receives the test report 24 hours later. Nucleic acid test is negative, and the validity period recognized by general institutions is 3 to 7 days. If people with negative test results come into contact with confirmed or suspected cases of pneumonia in COVID-19 again, or go to middle and high risk areas, they need to be re-examined.
The substance of nucleic acid detection is the nucleic acid of virus. Nucleic acid detection is to determine whether the patient is infected by COVID-19 by detecting whether there is nucleic acid of foreign invasive virus in respiratory tract samples, blood or feces. Therefore, once the nucleic acid is "positive", it can prove that there is a virus in the patient. The test results of nucleic acid in throat swab can be obtained in 6 hours at the earliest, but due to the large amount of detection at present, the examinee usually receives the test report 24 hours later. Nucleic acid test is negative, and the validity period recognized by general institutions is 3 to 7 days.
After COVID-19 infects the human body, it will first reproduce in the respiratory system, so we can judge whether the human body is infected with the virus by detecting the virus nucleic acid in sputum and nasopharyngeal swab. Therefore, positive nucleic acid detection can be used as the gold standard for the diagnosis of novel coronavirus infection. If people with negative test results come into contact with confirmed or suspected cases of pneumonia in COVID-19 again, or go to middle and high risk areas, they need to be re-examined.
Nucleic acid detection is also a method to detect virus, which can find infection earlier. Some respiratory diseases can be treated by throat swab. Or nucleic acid detection of respiratory secretions to determine whether it is infected or not, and to speculate whether it is contagious. Seasonal influenza, avian influenza, coronavirus infection, etc. It is easy to determine whether there are virus components in the upper respiratory tract by throat swab detection, so as to determine whether the detected person belongs to the infected person.
How to do nucleic acid detection?
At present, the method of nucleic acid detection is to take a throat swab (after the virus is infected, it will reach the oral mucosa epithelium, and the sampling purpose can be achieved by taking the throat swab), and send it to a special testing room within 1 hour. This nature determines that it is impossible for every patient to do it right away if he wants to. For example, patients who come in the middle of the night can't do it, so they can only wait until the instrument is turned on the next day.
Taking throat swab is a high-risk operation. According to the current clinical research, there are three modes of transmission in novel coronavirus, namely direct transmission, aerosol transmission and contact transmission. Among them, fog droplets are mixed in the air to form aerosol, which causes infection after inhalation. Therefore, there is a risk of infection for sampling personnel.
Result judgment:
If the test result is negative, the patient is relatively safe, but the danger cannot be completely ruled out (because the sample quality or the existence of inhibitors may lead to false negative results); If the test result is positive, the patient is in danger. The first and second steps have strict operating procedures and operating conditions. The overall time consumption is between 1-4 hours.
At present, the nucleic acid detection in Xiangya Hospital of Central South University usually starts at 1 1, and the results are reported around 16. During the period of nucleic acid detection and waiting for nucleic acid detection, patients can't leave the hospital at will.
If the nucleic acid is negative for two consecutive times and there are no clinical symptoms, the isolation will be further lifted according to the condition. If the nucleic acid is positive, mild cases can be isolated with household masks. We have prepared a proposal for home isolation, which is very detailed. Non-mild cases must be treated in isolation according to law.
Detection steps and principles
Detection steps:
1. First of all, the nucleic acid detection box used to detect novel coronavirus now mainly uses throat swab as the magic weapon in this box. Wipe the posterior pharyngeal wall and bilateral pharyngeal tonsils with a throat swab for 5- 10 times, and rotate the swab continuously.
2. The medical staff shall keep the sample, immerse the swab head in the cell preservation solution, and tighten the tube cover immediately after cutting off the tail.
3. Preservation: put the sample tube into a sealed bag for timely inspection. The transportation environment is strict and it should be kept at 2-8 degrees Celsius.
4. Perform nucleic acid extraction (reaction management), and extract nucleic acid from inactivated virus samples for subsequent detection.
5. Fluorescence PCR nucleic acid detection, that is, it takes 70-80 minutes to carry out fluorescence PCR amplification reaction on the extract through machine detection.
Nucleic acid detection is to determine whether the corresponding gene nucleic acid exists in the sample by detecting the accumulation of fluorescence signals. At present, most nucleic acid detection kits use fluorescence quantitative PCR. The detection principle is to take the unique gene sequence as the detection target, and through PCR amplification, the target DNA sequence we selected will increase exponentially. Each amplified DNA sequence can be combined with our pre-added fluorescent labeled probes to generate fluorescent signals. The more amplified target genes, the stronger the accumulated fluorescence signal. If the target gene is not amplified, the enhancement of fluorescence signal cannot be detected. Among the components of nucleic acid detection kit, detection primers and probes are the most critical. Sensitive and specific amplification primers play an important role in the accuracy of detection results.
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