Coronavirus

Still banging on about false positives?

Prof Spiegelhalter suggests the term is being misused and misinterpreted. The chances of a test showing that someone who doesn't have an infection as positive is <1%. Probably around 0.8% Testing a group of people is different and depends on how common the virus is amongst the group. Spiegelhalter uses the 1 person in 1000 example, which would return 9 positives. But goes on to say that this only occurs if the sample is random. In Pillar 2 testing, the sample is not random and the current positivity rate is 4.5%, suggesting that rather than 1/1000 having the virus, the level may be more like 40 to 50/1000. Spiegelhalter says: "In this case, even with a false positive rate of around 0.8%, most of the positive cases will be true positives". Also goes on to say that 0.8% false positives seems far too high and ONS data suggests something like 0.04 or 0.02% false positive rate.

Spiegelhalter calls the false positive issue a "distraction" and a "complete red herring.

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Originally Posted by Dave_Mole

Still banging on about false positives?

Prof Spiegelhalter suggests the term is being misused and misinterpreted. The chances of a test showing that someone who doesn't have an infection as positive is <1%. Probably around 0.8% Testing a group of people is different and depends on how common the virus is amongst the group. Spiegelhalter uses the 1 person in 1000 example, which would return 9 positives. But goes on to say that this only occurs if the sample is random. In Pillar 2 testing, the sample is not random and the current positivity rate is 4.5%, suggesting that rather than 1/1000 having the virus, the level may be more like 40 to 50/1000. Spiegelhalter says: "In this case, even with a false positive rate of around 0.8%, most of the positive cases will be true positives". Also goes on to say that 0.8% false positives seems far too high and ONS data suggests something like 0.04 or 0.02% false positive rate.

Spiegelhalter calls the false positive issue a "distraction" and a "complete red herring.

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One of many Zombie "facts" that refuse to die.

It is important to remember that laboratory testing verifies the analytical sensitivity and analytical
specificity of the RT-PCR tests. They represent idealised testing. In a clinical or community setting
there may be inefficient sampling, lab contamination, sample degradation or other sources of error
that will lead to increased numbers of false positives or false negatives. The diagnostic sensitivity
and diagnostic specificity of a test can only be measured in operational conditions.
Operational false-positives and false-negatives will have significant impact in the way we respond to
the COVID-19 pandemic. They will affect national surveillance, and the functioning of the UK trackand-trace programme. We have been unable to find any data on the operational false positive and
false negative rates in the UK COVID-19 RT-PCR testing programme.

and based on the regular contributions from some Profs since this paper was published in June, and a reply from Matt Hancock around a week ago to Sir Desmond Swayne in the Commons, they still do not know the operational figures for the testing - or perhaps they do, but don't want to tell us.

Anyone can pluck a figure out of the air - I've done it myself and been criticised for it - but 0.8% is a nice figure to pluck out - what Prof Spiegelhalter suggests is fair enough - but it applies to cases of RNA gfragments and not infections.

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Originally Posted by Dave_Mole

a "complete red herring.

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You just can't stop can you! ;)

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Originally Posted by Mossdog

You just can't stop can you! ;)

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I was referring to the false pollock-tives

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Originally Posted by Mike T

One of many Zombie "facts" that refuse to die.

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indeed.

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Originally Posted by Mike T

One of many Zombie "facts" that refuse to die.

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explain please Mike - false positives are an issue being increasingly raised over a period of 3 months. It hasn't gone away.

False positive rates are a lot smaller than false negatives rates. So the false positive rate is less than 1% whereas the false negative rate could be as high as 20%.

Clearly false negatives are more alarming, because it results in many people who have covid and are infectious being told they're fine and to go back to work/school etc.

False positive tests result in a very small number of people isolating when they needn't have.

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Originally Posted by noel

False positive tests result in a very small number of people isolating when they needn't have.

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False positives don't just affect the person who has it but everyone they've been in contact IF track and trace is working!!:rolleyes:

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Originally Posted by Wheeze

False positives don't just affect the person who has it but everyone they've been in contact IF track and trace is working!!:rolleyes:

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Yes good point. So if we ever get to 100,000 tests per day, that would be 800 false positives - although if you go on ONS data of 0.04% that's a lot fewer: only 40 people.

And if each of those had close contact with on average 10 people in the last few days, that's either 8,000 or 400 people isolating without needing to. And these are daily figures, so the cumulative total would add up as people might isolate for about ten days.

So it's definitely an issue. But not as serious as 20,000 people per day with COVID being told that they don't have it. If each of these people were in close contact with 10 people, that's a lot more worrying.