How to fake a lateral flow test

Soft drinks make lateral flow tests give a false positive result. Why?
12 July 2021

Interview with 

Mark Lorch, The University of Hull


Lateral flow test


There have been reports that kids have been using soft drinks to fake their COVID tests and get out of school. In fact, you can find out how to do it on TikTok, and apparently all you need is some orange juice or cola. But is it true? And if it is true, how does it work? Eva Higginbotham spoke to Mark Lorch from the University of Hull...

Eva - Many of us are familiar with lateral flow tests by now - the small plastic COVID tests that we can do rapidly at home, but fewer of us actually know how they work - myself included. I didn't know that they even have gold in them… How much money do you think I could make if I collect all the lateral flow tests I can find and take the gold out?

Mark - Uh, pennies, probably! That's a really good question. You know what? I might have to go and do the calculation to figure out...

Eva - That's chemist Mark Lorch from the University of Hull, and when he heard reports about people faking positive lateral flow tests using various soft drinks, he took it upon himself to figure out how on earth this works. But first, here’s how lateral flow tests are supposed to work...

After swabbing the back of your throat and nose and mixing the swab with a little buffer solution, you put a drop of the solution onto the sample window, otherwise known as S window, of the test. This liquid gets pulled along this absorbent strip of material inside the test, and along the way, will be exposed to specialised antibodies. Now, your immune system makes antibodies, these protein molecules that are highly specific in what they will bind to, in reaction to foreign invaders in your body, but scientists can also engineer antibodies to bind to whatever they like, and can stick colourful chemicals onto them too. So in this case, they made some antibodies that are highly specific to the coronavirus and are bound to little nanoparticles of gold which, surprisingly, look more purple and reddish than what we think of gold. If your sample contains coronavirus particles then those colourful antibodies will bind to the virus particles. These then get stuck to a second set of antibodies halfway up the test, at the T or Test window, which is why you would see a reddish-purple line there if you’ve tested positive. There’s a third set of antibodies involved too, stuck at the C or control line - these antibodies will bind to the other antibodies used in the test, and are put there to show you that the test has worked. If you get a control line and no test line, you have a negative test. If you have a control line and a test line, you have a positive test. But there are reports of kids faking positive tests using soft drinks. I wanted to see it for myself, so got Mark to talk me through it...

Okay. So I've got myself some orange juice. Unfortunately, it's got bits in, is this going to be a problem?

Mark - I don't know actually, I've never tested this with orange juice with bits in! So this is groundbreaking science going on here. Do you get an apparently positive result on a lateral flow test if you use orange juice with bits in?

Eva - Let's find out! All right. So I'm going to squeeze a couple of drops then onto the sample window just as I would if I was taking it myself. And I haven't mixed this with the buffer solution, this is just straight-up orange juice

Mark - What's happening now then is that orange juice is wicking up the device, it's picking up the red nano-particles attached to the antibody and it's carrying on the lateral flow test

Eva - And I can see a line where the T is! And it's actually, it's getting stronger as the seconds go by. It's just past the C now, and it's getting stronger there too. So there you go!

Mark - Well, there we go, you heard it here first - orange juice with bits in does give you a fake positive lateral flow test result for COVID

Eva - Right. So what is going on? Why does this happen?

Mark - Good question. Now, the thing that I've noticed is that all of the fluids that create this fake positive result are all acidic. So colas, orange juice, and so on, all have are about pH 3 and 4, so they're quite acidic solutions. Now the antibodies have evolved to work in very different conditions, they work in your bloodstream, which is about pH 7.3, so almost neutral, and the acidity of these drinks and so on is then quite an alien environment to the antibodies. Now antibodies are proteins and proteins are really quite sensitive to the conditions around them, particularly the pH, and by putting the antibodies in acidic conditions what essentially is happening is you're starting to unfold the antibodies. So antibodies, like all proteins, are long chain-like molecules that fold up into a very well-defined shape to do their job correctly. And when you put them in acidic conditions, you cause the antibodies to unfold slightly, which essentially destroys the specificity of the antibodies for the virus instead makes them stick to all sorts of other things in the solution, most obviously the gold nanoparticles and that's what results then in that line at the T.

Eva - So there you go - it comes down to the fact that antibodies often have quite a specific pH range that they operate at, much like how the enzymes in your stomach that help breakdown food work best in an acidic environment. And what if you want to check if a positive test is real or not? Mark told me that you can let it dry out over a few hours and then add some buffer solution to the sample window. This will wick up the test and, over time, reset the pH to about neutral causing the antibodies to get back into their proper shape. If the test is fake, the T line will disappear, and you'll know to start keeping a closer eye on the orange juice in your fridge.


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