Patrick Vallance clip

[fungussingstheblues]

Patrick Vallance is admitting in a clip I can't seem to attach that PCR tests can return positive results when a person isn't infectious.

It's from the daily briefing on 10 February.

What proportion of tests does this? Pretty crucial to know the answer, isn't it, considering the entire lockdown policy of the country has been based on these tests for more than a year now??

Hold on and I'll write the text verbatim.

[quote JellyBabiesSaveLives]This is a good explanation www.huffingtonpost.co.uk/entry/false-positives-coronavirus_uk[/quote]
No, that's not how it works - that huffpost picture is notoriously wrong as it 'readjusts' for symptoms. See my maths example.

Your maths example is mathematically correct TravellingTilbury but starts with three assumptions: A prevalence of 0.2%, a base false positive rate of 1%, and the people being tested having the same likelihood of infection as the rest of the community - i.e. not being picked for testing because they have symptoms.

None of those conditions apply to our current situation.

I can’t read the Huff Post article for some reason, but I am familiar with the BMJ calculator shown in the image and correction for the presence of symptoms is not an error, it is fundamental to the application of Bayes’ Theorem to medical testing. You start off with the prior probability of the person having condition X which is the population rate adjusted for the presence of symptoms.

If an MMR scan on body part A incidentally shows possible abnormalities in a patient’s lungs you proceed differently to how you would if a patient comes in coughing up blood and then gets the same scan. This is the bread and butter of medical testing, because all tests are imperfect. It is not something which the medical establishment have only recently discovered.

In the interests of completeness I should have said that the prior probability is the population rate adjusted for the presence of symptoms and also the individual’s risk factors. In a Covid example that might be known exposure to someone with the disease or a public-facing job. It doesn’t mean things like obesity which would put you more at risk once you got the disease.

Excuses - I agree and that is my point - that mass PCR testing in asymptomatic population (eg surge testing, moonshot etc) where prevalence is low has to be balanced against the risk of false positives. I'm not saying that PCR testing doesn't have it's place where there are symptoms or where the prevalence is high (although again, we have to be careful when we talk about prevalence - the prevalence is the actual sick people, not just positive test results, else it just becomes a circular argument).

Ideally, in the absence of clinical diagnosis, the PCR should be used when there are also symptoms for the result to meaningful.

The PCR test in itself it not the problem, it is the way it has been/can be used on a large scale which leads to more chance of figures with error and more chance that the figures can be manipulated by governments/councils etc.

Personally, I think, in the absence of symptoms, two difference tests should be used to mitigate the chance of false positives.

Remember, there are now many people who have compulsory testing (regardless of the absence of symptoms) who didn't previously have to test (eg for work, travelling etc). It's an important issue - and may become more increasingly important as deaths/prevalence of illness reduces. Shame on anyone jumping on the bandwagon and trying to shut down the issue without understanding the full implications and how they change over time.

I do love signal detection theory . A fascinating way of looking at things.

It is an issue potentially in the future in different circumstances.

It is not an issue in the circumstances in which we find ourselves now, as was clearly evidenced by the way that December’s wave of positive tests translated into hospitalisations and deaths.

The way in which Julia Hartley Brewer and others have implied that it is an issue that applies to us now is both ignorant and irresponsible.