A post shared widely on Facebook claims that an orange has tested positive for Covid-19 using a lateral flow test. It’s unclear whether the post is implying that the tests are inaccurate or unreliable. In reality, the test cannot work properly with an orange. The fact that these tests can effectively be broken with fruit does not make them inaccurate or unreliable for use in the general population.
Research shows that approved lateral flow tests are highly unlikely to give a false positive result when used correctly. We’ve written before about claims that a kiwi fruit and glass of Coca Cola have appeared to test positive with lateral flow tests.
Dr Alexander Edwards, associate professor in Biomedical Technology at the University of Reading, previously told Full Fact on this subject: “If you completely ignore the manufacturer’s instructions or in fact use the test for something completely different, then you shouldn’t really be surprised if you get a silly result.”
Annette Beck-Sickinger, professor Biochemistry at the University of Leipzig, told fact checking site AFP Fact Check that the buffer solution you mix with the sample in a rapid test would not be able to “neutralize large amounts of acid”, such as the amounts found in fruit like mangoes or apples.
The reliability of diagnostic tests is evaluated in experiments before they are approved. Early results show lateral flow tests rarely give false positive test results when properly used.
In the UK, the government is using a large number of Innova rapid tests. When trialled in labs and in real-world settings such as hospitals, schools and testing centres, they correctly identified negative samples 99.68% of the time.
Although these tests rarely give false positive results, they can produce false negatives. In its UK evaluation, the Innova test correctly identified 76.8% of positive samples, missing 75 out of 323. Some concerns have been raised about their ability to detect positive cases depending on who is administering the tests.
This makes rapid Covid tests controversial among some experts, who disagree about whether the benefits of quickly identifying lots of infected people will outweigh the risk of falsely reassuring others.