Please refer to this previous blog post if you would like a deeper dive into all this terminology.
To assess the result of your lateral flow test we need to know three things.
The sensitivity of the test (how good it is at telling you you’ve got the virus when you actually do have the virus).
The specificity of the test (how good the test is at telling you that you don’t have it when you truly don’t).
And the prevalence or incidence rate of the illness in the population.
The following calculations assume you are asymptomatic. None of this applies if you actually have symptoms. If you have symptoms, isolate and get a PCR test.
So, for DIY lateral flow tests the sensitivity is roughly 65.0%.
And for the specificity, it’s about 99.9%.
The incidence rate has improved from a really poor 1 in 50 back in early January to over 1 in 1,000 now. We’ll assume it’s 1:1000.
On that basis, if you get a positive test result from the lateral flow test, it’s only 40% likely to be correct. It’s more likely (60%) to be a false alarm.
Still, you should isolate, get it confirmed – if you can – by a PCR test, or at least – while isolating – repeat your lateral flow test.
If you get a negative result, rest assured. It’s hugely likely to be correct.
PS: if the incidence rate improves even further to, say, 1:1500, then a positive result from a lateral flow test becomes only 25% likely to be correct – it’s 75% likely to be a false alarm.
And remember, ignore the above logic if you actually do have symptoms.