Why does the new coronavirus nucleic acid test appear "false negative"? Will it lead to missed diagnosis?

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The new crown virus swept through at the beginning of 2020. No matter how to prevent and control it, it can always escape cunningly and suddenly appear. Some time ago, the epidemic in Beijing had just been brought under control, and news of the closure of the city came from Urumqi, and now Dalian has been infected again. Another cunning manifestation of it is that it cannot be displayed during the test, which leads to the existence of "false negatives". Desheng will answer you why the new coronavirus nucleic acid test has "false negatives"? Will it lead to missed diagnosis?

Expert response: "false negatives" are inevitable

The State Council’s Joint Prevention and Control Mechanism held a press conference. In response to the "false negative" in the nucleic acid test of patients with new coronary pneumonia, Gao Zhancheng, director of the Department of Respiratory and Critical Care Medicine, Peking University People's Hospital, responded: The detection rate of any viral nucleic acid test cannot be 100%, and the nucleic acid test of the new coronavirus No exception, "false negatives" are inevitable.

Expert Gao Zhancheng pointed out that the detection rate may also be related to many factors such as the severity of the patient's condition, the development stage of the disease course, sample collection and laboratory testing conditions.


Question: Why are there "false negatives"?

Answer: For example, the concentration of viral load in the respiratory tract of a patient is closely related to the disease process. Sampling at different times may result in different results. In the early stage of the disease, the virus is less, and it is also difficult to collect.

The lesions of new coronary pneumonia are mainly in the deep part of the lungs, far away from the atmospheric duct, and the infected people have more dry coughs and not much sputum. The virus is more difficult to stay in the upper respiratory tract, and the positive rate of sampling in the upper respiratory tract will be relatively low. However, it is currently difficult to sample the lower respiratory tract of every patient.

In actual operation, simpler and faster throat swab sampling is often used. In the early stage, many patients have dry cough and no sputum, which makes sample collection and detection difficult.

According to the technical guidelines for laboratory testing of new coronary pneumonia issued by the National Health Commission, there are strict requirements on the types, methods, packaging and preservation of specimens collected. Similarly, every link will affect the test results.

The guidelines state that acute respiratory tract specimens and acute blood specimens must be collected for each case, and lower respiratory tract specimens (such as bronchial or alveolar lavage fluid, etc.) must be collected for severe cases. Upper respiratory tract samples include throat swabs, nasal swabs, and nasopharyngeal aspirates. Lower respiratory tract samples include deep cough sputum, respiratory tract aspirates, bronchial lavage fluid, etc.

The guidelines specifically point out that a negative result cannot exclude new coronavirus infections, and factors that may produce false negatives need to be excluded, including: poor sample quality, such as respiratory samples from oropharynx; samples collected too early or too late; incorrect The preservation, transportation and processing of samples; the reasons for the technology itself, such as virus mutation, PCR inhibition, etc.

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To diagnose new coronary pneumonia, nucleic acid testing is an indispensable means

Q: If there is a "false negative", should I give up the nucleic acid test?

Answer: Expert Gao Zhancheng said that nucleic acid testing is currently indispensable for the diagnosis of new coronary pneumonia. Nucleic acid testing is a faster and easier method than genetic testing, and it has an irreplaceable position in the face of the epidemic.

Earlier, there were calls on the Internet to use CT instead of nucleic acid testing. In fact, in the "New Coronary Pneumonia Diagnosis and Treatment Plan (Trial Fifth Edition)", imaging has been added as a clinical diagnostic standard, but nucleic acid testing is still required for diagnosis.

Because CT can only determine that a patient may have viral pneumonia, it is impossible to determine whether it is caused by the new coronavirus. Using CT instead of nucleic acid testing may cause misdiagnosis, causing the misdiagnosed person to be infected by the infected person during isolation.

Nucleic acid testing is used as a basis for pathogen diagnosis and the most important basis for diagnosis at present. It is the "gold standard" for diagnosing new coronary pneumonia.

Q: Will "false negatives" cause a large number of patients to miss a diagnosis?

Answer: Don't worry too much. Nucleic acid test is used as a standard for confirming the new coronavirus, but a negative nucleic acid test is not a basis for excluding infection. Patients who have not been diagnosed will be classified as suspected patients. During the follow-up diagnosis and treatment of suspected patients, nucleic acid testing and differential diagnosis will be performed many times, and the final diagnosis result will be obtained. In other words, nucleic acid testing is not the only criterion. Doctors will also analyze the clinical diagnosis and the specific conditions of the patient. For example, for patients with highly suspected symptoms and imaging features, even if the test is negative, further observation is needed.