A post on Facebook falsely claims that Covid-19 tests have a 100% false positive rate, because the virus causing the disease cannot be detected with tests. This is completely untrue.
The post shares a screenshot that says: “I want you to truly understand that currently there is a 100% #False Positive Rate because what the PCR is looking for is something that nobody knows.” The screenshot includes a picture of a Covid test (which in fact looks like a lateral flow test, not a PCR test).
The post also includes a long passage of text, but this article will only consider the image itself.
We’ve written before about how PCR tests work, and why they are an accurate way to detect the Covid virus. These tests detect the virus’s RNA (genetic material) in a sample, usually a nose or throat swab, from the person tested. Enzymes are then added to make copies of any viral RNA present, which can then be detected when the sample is tested.
It is also very clear that Covid tests do not have a 100% “false positive rate”, in the usual meaning of that phrase. If a test had a false positive rate of 100%, it would mean that it gave an incorrect positive result every time. The vast majority of Covid tests give a negative result.
The exact false positive rate of PCR Covid tests is extremely small. The Office for National Statistics (ONS) says that the accuracy of its positive tests “must be very close to 100%”. Even if every single positive test in the ONS Infection Survey was false during a six-week period in summer 2020, the false positive rate of the tests would be just 0.08%.