(Reuters) - Social media users claiming that fruit juices and coffee have tested positive for COVID-19 have misunderstood how lateral flow devices (LFDs) work. Videos allegedly showing a positive test result actually reveal what happens when devices are degraded by using substances other than those intended.
Examples of the claim can be found here:
“Imagine just how many hundreds of thousands of fake positives have been reported as actual covid,” one individual said in a Facebook post.
Many of the users have tested LFDs with substances other than saliva to prove they are ineffective. They include examples of the devices showing two pink lines, indicative of a positive COVID-19 result.
However, this is misleading.
“The beverages and other solutions used do not contain the COVID virus,” Professor Mark Lorch, professor of public engagement and science communication and interim head of department for chemistry, biochemistry and chemical engineering at the University of Hull, told Reuters by email.
He said that while results might appear to be positive, they reveal something else. “Instead, the acidity of the juices, soft drinks, coffee etc. disrupt the delicate antibodies on the test devices and so corrupt the test results.”
Inside a lateral flow test, there is a strip known as the conjugation pad where antibodies are attached to gold nanoparticles. Those antibodies bind with virus particles should the test encounter the coronavirus (Lateral Flow Test Teardown). After taking a swab from the throat and nose, the sample is mixed with a buffer to ensure optimum pH before dripping it onto the strip.
“These tests rely on molecular components such as the antibodies and functionalised nanoparticles being able to bind and this is determined by electrostatic interactions between the components,” Andrea Sella, a professor of chemistry at University College London (UCL), told Reuters.
“When it comes to biological components, most of them have components that are very pH sensitive. For example, if you change the pH, you can completely change the charge of a protein from, say, positive to negative and the result is that it will not stick correctly.”
Sella added that it was therefore “unsurprising” that LFDs were disrupted when testing substances such as Coca-Cola and orange juice, considering the pH is significantly more acidic than biological fluids.
The two resulting red lines on an LFD do not have any meaning in these circumstances, he said, because “you’ve wrecked the underlying chemistry that allows you to do the delicate detection you want.”
Moreover, the US Food and Drug Administration (FDA) has warned that there is potential for false positive results with antigen tests, including when users do not follow the instructions.
Reuters previously fact-checked the claim that Coca-Cola produces a positive COVID-19 test.
Thus, Fruit juices and coffee do not test positive for COVID-19. The acidity in such substances corrupts lateral flow devices.