Thread by @dgurdasani1

Hearing many reports of children/adults who've been LFD+ve and PCR -ve for SARS-CoV-2. Given current prevalence the probability of having infection if an LFD test is positive is extremely high - e.g. 97% for adolescents. 🧵

theguardian.com/world/2021/oct…

The calculations above outline the overall positive predictive value (chance of actually having infection if one is positive) at a prevalence of 7% (based on ONS prevalence in secondary school children). This shows that with a positive test, probability of infection is 97%

For groups (e.g. adults) where prevalence is lower (~1%), the positive predictive value (PPV) drops but is still pretty high, at 83%. The exact PPV will depend on prevalence of infection which will differ by region and age group, but is generally high at the current time.

In symptomatic people, or contacts of a case, this probability is even higher with a +ve LFD. It's important to understand that govt policy that those who are LFD+ve & PCR-ve don't need to isolate was based on low prevalence scenarios where positive predictive value was lower.

So please do isolate if you have a +ve LFD & if you can, even if PCR -ve. You are likely to be infected, and be able to transmit to others. +ve LFDs and -ve PCRs can happen for different reasons, including quality of swab, type of swab, technical issues.

The govt should revise it's policy to bring it in line with current prevalence - which makes a policy that asks people with +ve LFDs to go back into work or schools based on -ve PCRs flawed, and likely to worsen spread.