On June 24, a school in England warned the parents of its students about false-positive covid tests and asked them to be vigilant when their children were taking Lateral Flow Test (LFT) for the coronavirus. The school had discovered that some students faked positive covid tests using fruit juice. The school claimed that the methods were being shared on social media channels. The school had to stop kids of four groups from attending till the issue was resolved.
To find how the kids were able to break the test to get a positive result, a professor of science communication at the University of Hull in the United Kingdom, Mark Lorch took it upon himself to expose the secret behind the trickery. In an explainer that he published in The Conversation, Lorch explains the method in detail. To understand how the test breaks, we need to understand how it works. The LFT, when opened, consists of a narrow pad with four sections on it. The first section is where the sample is put. The second section has a reddish outlook, thanks to the tiny gold particles attached to otherwise invisible antibodies. The next two sections are a test section and a control section. The test section, having primary antibodies that bind to the virus, is supposed to trap the virus stained by the gold-coloured antibodies. The section shows a line if the virus is detected. The control section traps the stained antibodies to prove that the test is working and antibodies actually crossed the test section.
The test entirely relies upon the antibodies' ability to bind themselves with the virus. The secret lies in the fact that antibodies work largely around the pH of the bloodstream, which is neutral. Now, when acidic juice from fruits such as lemon and orange, is put on the test, it breaks the antibodies' ability to perform as expected. Antibodies, which are fundamentally complex proteins, are broken down into denatured simpler ones that are sticky. As a result, the stain sticks to the test region showing a false positive.