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Why Antigen Testing is Not a Substitute for Nucleic Acid Testing丨COVID-19 Stories

By VIVIAN YAN|Mar 22,2022

Chongqing- What is COVID-19 antigen testing? Who needs to be tested with the antigen self-test kits?

Liao Pu, deputy director and chief technologist of the Chongqing Center for Clinical Laboratory, Chongqing Municipal People's Hospital, answered questions about the public's concern regarding COVID-19 antigen testing on March 20.

Photo of the new COVID-19 antigen detection kit.

Photo of the new COVID-19 antigen detection kit. (iChongqing file photo)

Q: What is antigen testing, and what are its main features and advantages?

A: The antigen test for COVID-19 is quick and easy to perform. Instead of relying on professional institutions, equipment and personnel, people can collect their own nasal samples with the kits, and the results are usually available in 30 minutes.

By implementing antigen testing on relevant and key populations, people who test positive with the antigen kits can be quickly screened out, helping to curb the spread of the epidemic.

The COVID-19 antigen test commonly uses colloidal gold immunochromatography assay (GICA), which detects the N antigen of COVID-19, a specific protein that exists on the outer shell of the virus itself by an antigen-antibody binding reaction.

In general terms, GICA is an effective screening method for identifying detectable anti-SARS-CoV-2 neutralizing antibodies.

Q: What are the main differences between antigen testing and nucleic acid testing?

A: Their sensitivity and specificity are different. If a person is infected with COVID-19, the SARS-CoV-2 virus can only be detected when the content reaches a certain level in the pharynx.

For antigen testing, if the virus reaches one million or even 10 million per ml, the person's COVID-19 antigen test result can be positive. Nucleic acid testing has an exponential amplification process. It is 100 to 1000 times more sensitive than antigen testing.

The specificity of antigen testing can reach 99%, while nucleic acid testing can reach 100%. Therefore, nucleic acid testing has been the gold standard for COVID-19 infections.

This means that antigen testing is not a substitute for nucleic acid testing. When the antigen test result is positive, it must be reconfirmed with nucleic acid testing.

Q: Who needs to be tested with the antigen self-test kits?

A: The antigen self-test kits are suitable for high-risk groups, including patients who go to primary-level medical centers with respiratory and fever symptoms in five days and people who are under quarantine. They will be given the self-test kits by the relevant professional bodies in accordance with the relevant national regulations.

Antigen testing is generally not necessary for the general public.

If people are concerned about being infected due to some risk of local transmission of the epidemic, or if people are personally concerned about being infected due to exposure to risk groups, environments, items, etc., they can purchase antigen testing reagents online or at retail pharmacies for self-testing.

It is important to note that if the result of an antigen test is positive, all used swabs, tubes, test cards, etc., must be placed in sealed plastic bags and handed over to the staff for disposal as medical waste.

A medical worker took a swab sample from a man for a COVID-19 nucleic acid test. (iChongqing/ Zhang Deng)

(Shi Jiaqi, as an intern, also contributed to this report.)


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