Reports that UK students were using various drinks to get fake COVID-19 test results prompted University of Liverpool researchers to study the effects of applying soft drinks and artificial sweeteners to the test swabs.

Teenagers had figured out how to use soft drinks to fake a positive COVID-19 test, and the authors of the new study are warning schools and other groups to be aware.

During the first week of July, videos uploaded to social media with the hashtag #fakecovidtest showed teenagers applying various liquids to rapid antigen COVID-19 tests, garnering millions of views, according to several media reports.

All four sweeteners tested by the researchers produced negative results on rapid COVID-19 tests, as did spring water. But 10 of 14 soft drinks produced positive or weakly positive results, with no apparent link between the test results and the soft drinks’ ingredients, the researchers reported on Monday on medRxiv ahead of peer review.

Since March, UK schools have asked pupils without symptoms to test twice weekly, the authors note. A positive test can result in an entire class having to isolate at home. Based on their findings, they advise, testing “should be performed first thing in the morning, prior to the consumption of any food or drinks, and supervised where feasible.”