Soda and lemon juice cause false positives in rapid COVID-19 tests



VTikTok iral videos showing teens hacking rapid COVID-19 tests by inserting sodas and other acidic solutions into the test entrance are causing a stir, especially in the UK, where these easy-to-use home tests are currently used for regular screening of high school and college students and teachers.

While PCR tests remain the gold standard for detecting SARS-CoV-2, so-called rapid COVID-19 tests instead use lateral flow technology and can return a result in 30 minutes or less.

In addition to their speed, these rapid tests can also be performed anywhere, increasing the accessibility of the tests. However, preliminary research published on the preprint server medRxiv July 5 confirms that certain drinks can cause false positives. And because anyone in the UK who gets a positive rapid test result must immediately self-isolate for up to 10 days, report the result and follow up with a PCR test – or face a fine – the UK education officials are concerned that such positives are being used to leave school. Geoff Barton, general secretary of the Association of School and College Leaders in the UK, told the newspaper I that such piracy is limited to “a very small minority of pupils”.

The sudden explosion of quick test hack videos added to existing concerns about their accuracy. “People say ‘well, these tests are hopeless,'” says Alexander Edwards, who researches and develops clinical and microbiological tests at the University of Reading School of Pharmacy in the UK.

See “COVID-19 Diagnosis: How Do Saliva Tests Compare to Swabs?” ”

The scientist spoke with Edwards to understand how sodas can cause false positives and the broader implications of this type of test hacking, as well as why these tests are not hopeless.

Courtesy of Alexander Edwards

The scientist: What is a lateral flow test and how does it work?

Alexandre Edwards: A lateral flow test is a quick way to measure a particular target, which is useful for clinical diagnosis, in a complex mixture. And that mixture could be something from a swab, or it could be blood, or something like that. The way it works is that it binds to that target with the help of antibodies. And if the target is present, the antibodies in the system allow you to get a visual line, which is your positive line, because the antibodies and antigen trap the target on that band on that little line.

ST: How precise are they?

AE: Lateral flow assays can be extremely precise and are very powerful at detecting particular targets in complex mixtures. The only catch is that they are as precise as the way they are used.

The great advantage of lateral flow assays is that they are portable, inexpensive, disposable, and easy to use near the patient, but this comes at a small cost in terms of analytical performance. . . . . The good news is that lateral flow tests can be extremely specific, and that means if you get a positive result it is very likely to be an actual result. What we mean by this is that we don’t tend to get a large number of false positives for COVID-19 side flow testing.

The manufacturers would say it’s simply because you have these gold particles, which make up the colored element, and if they are treated with an acidic solution, they can clump or clump together.

There is one final challenge regarding the accuracy of these tests, which is. . . sometimes tests are less accurate when used in a different way. And a really good example of this is that if you take a test and use it in a trained supervised environment, you tend to get very repeatable and reliable results – they work the same every time. But if you then deploy it in a community setting, when the people who use them might not be as experienced, you start to see more false negatives and more false positives, sometimes just because the people doing the test can make an error. , the instructions may not be very clear, or even something really silly, like the test is out of date, or it is just broken somehow.

ST: Why can something like sodas return a false positive result?

AE: The biochemistry is quite interesting, as it seems there is something strange at first glance. You might say, “Well, you should only be able to get a positive result if there’s the virus particle there, because that’s the point of the test; it should only be able to detect the virus particle. But there are some ways that these tests can go wrong, and one of them is really obvious if you put something in the test that isn’t supposed to go, especially something like a strong acid or an acidic solution. What you can do is mutilate, if you wish, or spoil the antibodies in the test. And when you do that, you can get unexpected results.

What I’ve been able to see — and there are even articles that have looked at this on an experimental basis — is that under certain circumstances you can get the appearance of a positive line. The manufacturers would say it’s simply because you have these gold particles, which make up the colored element, and if they are treated with an acidic solution, they can clump or clump together. This makes it sticky, and then it sticks where it shouldn’t be.

Anytime you get a false positive, especially if it’s a false positive resulting from misuse of the test, you are actually damaging it. . . public health system that is there to protect us.

To be fair to the manufacturer, they never intended these things to go into the test. So this is definitely not a weakness or a criticism of the test, because if you follow the instructions that come with a test, and don’t put in soda or a slice of kiwi fruit or whatever that people have tried on the test, and you actually follow the rules and take the test thirty minutes after eating and drinking, if that’s what the instructions say, then you won’t get these unexpected results.

ST: Why could false positives, whether intentional or not, be a problem?

AE: If you have a testing program where you test people most of the time. . . especially if you are testing people without symptoms, every time you get a false positive the person involved as well as anyone who has come in contact with them should be treated as if they have this very dangerous and highly contagious infectious disease. In many countries it is a legal obligation to report this finding, as the pathogen is considered very dangerous. . . . So every time you get a false positive, especially if it is a false positive resulting from misuse of the test, you are actually damaging the system, and the system is the public health system which is. there to protect us. This is obviously a bad thing.

ST: What can be done to minimize the impact of false positives?

AE: The advice is really very simple. . . . Try to follow the instructions that come with the kit as closely as possible. And the reason I say this is because when we have data to trust the test, that data is always generated when using the test according to the manufacturer’s instructions. So it’s possible that you might change the way you test, and it would still work fine, but we don’t know.

It sounds a bit boring, just follow the instructions. . . . But since this is a medical test, it is actually very, very important that you follow the instructions or the system has a tendency to crash.

Editor’s Note: This interview has been edited for brevity.


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