Why do some people have a fever but the antigen test is still negative? 6 questions about antigen testing, one article to understand

Why is the antigen still negative when someone has a fever? When is the most accurate time to test? What is the situation when the antigen turns negative and then "re-positive"? Focusing on the hot issues that everyone is concerned about, the reporter sorted out the professional answers from relevant experts of the Joint Prevention and Control Mechanism of the State Council and Zhang Boli, an academician of the Chinese Academy of Engineering.

01

Why is the antigen still negative when I have a fever?

When is the most accurate time to measure?

Antigen testing is fast and easy to operate, but antigens can only be detected when the virus replicates to a higher level and the infected person excretes a higher amount of virus.

In the early stage, even if the infected person has some mild symptoms, the antigen test may not show positive due to the low viral load. According to the incubation period of Omicron, it can generally be detected 2 to 3 days after infection. Some infected people, especially asymptomatic ones, may not be detected until 5 days or even longer due to reasons such as the continued low viral load.

02

Antigen test negative

Does it mean there is no infection?

If the virus has not replicated and been excreted in large quantities in the body, it cannot reach the sensitivity of the antigen test, and the result will be negative. Therefore, some people may have been infected and even have some mild symptoms, but the antigen test still shows negative. However, the viral load is usually lower at this time and the infectiousness is weaker.

Antigen testing does not require "twice a day" or "three times a day". If necessary, testing once a day is sufficient.

03

Why are some people antigen positive?

The nucleic acid test is negative?

The two detection methods have different sensitivities, and the sampling operation may not be standard, which may result in "false positive" or "false negative" results. Generally speaking, nucleic acid testing has a higher accuracy rate.

If the symptoms of the person receiving home treatment have significantly improved or there are no obvious symptoms, the self-test antigen is negative and two consecutive nucleic acid tests are negative (Ct value ≥ 35, and the interval between the two tests is greater than 24 hours), the home treatment can be terminated and normal life and going out can be resumed.

04

Why do people take antigen tests?

Turning negative and then "re-positive"?

Zhang Boli, an academician of the Chinese Academy of Engineering, said that re-infection and reinfection are two different concepts. The former is that the infected person has a positive result twice in the same infection cycle, while the latter is that the infected person has experienced different infection cycles.

One reason for re-positivity may be a false negative antigen test; another reason may be that the virus fragments brought out by the sputum plug presented a positive result. In the later stages of the infection, due to the low viral load, the antigen test may give a false negative result, so it is generally recommended to self-test the antigen for two consecutive days, or use nucleic acid testing as a judgment standard.

Excluding misdiagnosis and false positives, another possibility is that the patient's sputum is relatively viscous. Due to the viscosity, sputum plugs are formed, and the virus is wrapped in the sputum plugs. When the recovered person excretes the sputum plugs during the subsequent lung repair process, the virus is brought out. At this time, the positive result is caused by viral nucleic acid fragments, and live viruses cannot be cultured.

Regardless of the cause of the reinfection, clinical observations have confirmed that the reinfected cases did not transmit the virus to others.

05

What should I do if the antigen test is positive?

If the antigen test is positive, and there are no symptoms or the symptoms are mild, the patient should be isolated at home for treatment. According to the medication guidelines for home isolation treatment personnel, appropriate drugs should be selected for treatment. If the symptoms worsen, the patient should go to the fever clinic (examination room) of the medical institution in time. If the antigen test is positive for the elderly, the patient should also choose drugs under the guidance of medical staff. If the symptoms worsen, the patient should go to a tertiary hospital in time.

06

What should I pay attention to when purchasing an antigen test kit?

Check the production date and expiration date; check whether it has the medical device registration certificate number and complete UDI code (unique medical device identification) approved by the National Medical Products Administration; check whether the product composition is complete, including: reagent plate (the minimum usage unit must have a digital code), lysis solution, disposable sampling swab, instruction manual, and sealed bag.

Source: Zhejiang Daily

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