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Expert Explains Why Some People Testing Negative in Two Nucleic Acid and Antibody Tests Are Still Confirmed with COVID-19?

2020/07/02

How come some confirmed cases tested negative in the first two nucleic acid tests (NAT) and antibody tests?

At the 138th press conference on Beijing's COVID-19 response held on 1 July, Wu Zunyou, chief epidemiologist at the Chinese Center for Disease Prevention and Control, explained that NAT and antibody tests are designed to detect two different elements. NAT checks whether the virus has nucleic acid. A positive result indicates an active infection. Antibody testing detects the substance in blood produced in response to the virus. Antibodies are usually produced two weeks after infection. In the early stage of infection, antibodies are hard to be detected. This shows the two tests focus on two different aspects.

How come a confirmed patient tested negative in two NAT and antibody tests? As explained earlier, at the early stage of infection, antibodies are usually not yet produced, thus it is normal for antibody tests to yield negative results. In terms of NAT tests, the following three reasons may explain the two consecutive negative test results. First, after a patient is infected with COVID-19, the virus mainly stays in the lungs. Airways in the lungs may be blocked by virus-induced inflation, making it hard for the virus to reach nasopharynx where test samples are collected. In this case, NAT tests will not be able to detect the virus. Second, the swab may miss the virus if sample collection is not done properly. Third, patient conditions and the timing of sampling may also be a factor. As people respond differently to COVID-19 infections, the timing of sample collection and the interval between sample collections may also affect test results. All the three abovementioned scenarios may lead to false negatives.

Clinically speaking, there have been cases that tested negative first and were later confirmed, but such cases are rare. The confirmation of cases should not be based solely on lab results, but on a combination of factors, including epidemiological history and clinical symptoms. There is no need to worry about a false negative if the person testing negative has neither travel history to high-risk areas nor contact history with confirmed cases.

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