Understanding how the tests are conducted is important. You’ll see a strip called nitrocellulose and a small, red pad hidden beneath the plastic casing of the LFT device. The red pad contains antibodies that bind the COVID-19 Virus. The antibodies are also attached to nanoparticles, which appear as red particles. You must mix the sample with the liquid buffer solution to ensure that the pH is optimal before you drip it onto the strip.
Fake positive results. Mark Lorch is the author.
Fluid wicks the nitrocellulose up and picks the gold and antibodies. If the virus is present, these antibodies will also bind it. More antibodies bind to the virus further up the strip. They are located next to the T for test. These antibodies can’t move because they are attached to the nitrocellulose. The second set of antibodies also grabs the virus as the gold-labeled antibodies are passed over the red smear. The virus then binds to both antibodies, leaving the gold immobilized next to the T, which indicates a positive test.
Gold antibodies that haven’t bound to the virus carry on up the strip where they meet a third set of antibodies, not designed to pick up COVID-19, stuck at the C (for control) line. These trap the remaining gold particles, without having to do so via the virus. This final line is used to indicate the test has worked.
Acid test
So how can a soft drink cause the appearance of a red T line? One possibility is that the drinks contain something that the antibodies recognize and bind to, just as they do to the virus. But this is rather unlikely. The reason antibodies are used in tests like these is that they are incredibly fussy about what they bind to. There’s all sorts of stuff in the snot and saliva collected by the swabs you take from the nose and mouth, and the antibodies totally ignore this mess of protein, other viruses and remains of your breakfast. So they aren’t going to react to the ingredients of a soft drink.
A much more likely explanation is that something in the drinks is affecting the function of the antibodies. A range of fluids, from fruit juice to cola, have been used to fool the tests, but they all have one thing in common – they are highly acidic. The citric acid in orange juice, phosphoric acid in cola and malic acid in apple juice give these beverages a pH between 2.5 and 4. These are pretty harsh conditions for antibodies, which have evolved to work largely within the bloodstream, with its almost neutral pH of about 7.4.
Maintaining an ideal pH for the antibodies is key to the correct function of the test, and that’s the job of the liquid buffer solution that you mix your sample with, provided with the test. The critical role of the buffer is highlighted by the fact that if you mix cola with the buffer – as shown in this debunking of an Austrian politician’s claim that mass testing is worthless – then the LFTs behave exactly as you’d expect: negative for COVID-19.
So without the buffer, the antibodies in the test are fully exposed to the acidic pH of the beverages. And this has a dramatic effect on their structure and function. Antibodies are proteins, which are comprised of amino acid building blocks, attached together to form long, linear chains. These chains fold up into very specific structures. Even a small change to the chains can dramatically impact a protein’s function. A network of many thousands of interactions between the various parts of the protein maintains these structures. For example, negatively charged parts of a protein will be attracted to positively charged areas.
But in acidic conditions, the protein becomes increasingly positively charged. As a result, many of the interactions that hold the protein together are disrupted, the delicate structure of the protein is affected, and it no longer functions correctly. In this case, the antibodies’ sensitivity to the virus is lost.
Given this, you might expect that the acidic drinks would result in completely blank tests. But denatured proteins are sticky beasts. All of those perfectly evolved interactions that would normally hold the protein together are now orphaned and looking for something to bind to. So a likely explanation is that the immobilized antibodies at the T-line stick directly to the gold particles as they pass by, producing the notorious cola-induced false positive result.
Is there a way to spot a fake positive test? The antibodies (like most proteins) are capable of refolding and regaining their function when they are returned to more favorable conditions. So I tried washing a test that had been dripped with cola with buffer solution, and sure enough the immobilised antibodies at the T-line regained normal function and released the gold particles, revealing the true negative result on the test.
