Orange Juice-Cola Cause False Positives. (PHOTO:VCG)

By Zhang Jiaxing & BI Weizi

Recently, a social debate has been making waves online after people found that inserting orange juice or cola into the COVID-19 antigen tests' inlet can result in a positive test result. Some netizens caused an uproar, claiming false positive results can be produced too easily. However, from the perspective of researchers, these so-called "false positives" are completely false.

"It is common sense that any test product should be used in accordance with the instructions in order to get accurate results, "Xu Lei, director of research and development at Huada Yinyuan Pharmaceutical Technology, told Science and Technology Daily, adding that though some antigen detection products are accessible out of the laboratory, the process of testing and verification is ultimately a rigorous scientific operation.

Zhu Yuqing, professor in Department of Clinical Chemistry and Immunology, Shanghai Center for Clinical Laboratory, conducted a validation test on the above operations. She found that some test kits can screen out "bad tests" and show as invalid, others will be "masked"and show as weak positive.

So why would orange juice and cola cause false positives?

A true positive result appears when the antigen test captures the virus protein, and the antibody-virus protein binding reaction occurs at the test line and stimulates a chromogenic reaction, with both the quality control and test lines showing colors.

The orange juice and cola also allow you to get a positive test line because the two acidic solutions directly stimulate the chromogenic reaction without the presence of viral proteins. In chemical reactions, the pH within the solution system is critical and determines the dissociation and binding of ions.

In the design of the antigen test kit, buffer solution is employed to avoid the pH change. This solution acts as a buffer against small pH fluctuations brought about by sweat, air, etc., to avoid drastic changes in the entire reaction system. Even so, the buffer solution cannot "buffer" the damaging effects on the overall experimental results caused by the addition of large amounts of artificial acids.

It's not the antigen test that is prone to "false positives," but is it not being used in accordance with instructions, so the blame is not the orange and cola in this case.