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PCR 97% false positives Portugal court ruling

(14 Posts)
Taciturn Thu 19-Nov-20 08:14:12

"if someone is tested by PCR as positive when a threshold of 35 cycles or higher is used (as is the rule in most laboratories in Europe and the US), the probability that said person is infected is <3%, and the probability that said result is a false positive is 97%"

The article with links to academic articles is here:
medicalkidnap.com/2020/11/18/portuguese-court-rules-pcr-tests-as-unreliable-unlawful-to-quarantine-people/
tapnewswire.com/2020/11/portuguese-appeals-court-deems-pcr-tests-unreliable/

OP’s posts: |
thingsarelookingup Thu 19-Nov-20 08:21:57

So how did Victoria get down to 0 positive cases then?

QueenStromba Thu 19-Nov-20 08:37:45

That's massively twisting the data.

www.sciencemag.org/news/2020/09/one-number-could-help-reveal-how-infectious-covid-19-patient-should-test-results

Taciturn Thu 19-Nov-20 08:38:22

I've no idea. Fewer than 35 cycles? Not testing?
Victoria would also be in summer season so fewer seasonal viruses.
How did NZ suddenly have positive cases?

OP’s posts: |
TheLovleyChebbyMcGee Thu 19-Nov-20 08:41:17

The cycle number is taken into consideration in our lab for PCR tests and a lab comment is added if required. Also, not all labs use PCR, especially in the UK.

SexTrainGlue Thu 19-Nov-20 08:47:01

Thanks for the link @QueenStromba

It is staggering how science is so misreported.

For those who don't want to plough through the links, the short version is that lower CT score = higher viral load.

A positive at 35 CT is still a positive, but with a much lower viral load that if it was 20 or 30. Exactly where the cut off is (to return a positive when the person is going to be infectious) is something that is being kept under review.

Cornettoninja Thu 19-Nov-20 08:52:30

@SexTrainGlue (mind boggling username btw grin) thank you for your explanation but is there any chance you could dumb it down a bit more for my friend who is still a bit confused... so is this ‘97% false positive’ headline bollocks or does it hold some merit?

SexTrainGlue Thu 19-Nov-20 09:04:38

It's bollocks

They are genuine positives at 35 CT (well, nearly all of them are - any machine operated by a human can throw up the odd wrong result). The machine has accurately detected the virus after that number of cycles

The underlying issue is whether 35 CT is the correct diagnostic threshold.

Note that this is only about the 35 CT threshold. Not all the tests which return a lower number

Taciturn Thu 19-Nov-20 09:34:10

It is staggering how science is so misreported.
It's bollocks

You are quite mistaken @SexTrainGlue

It is a report about a portguese appeals court - with a quote directly attributed to that court - concerning PCR testing in Portugal.
It is NOT a science report at all, let alone a misreporting of one

You can claim that the court has the science wrong.
You could also claim that the report is fabricated (although I did try to verify this before posting), or that the quote is mis-translated or mis-attributed.

But at this stage it is a finding of fact that a Portuguese court made this claim. Incidentally, the ruling in favour of the plaintive was not depending on the finding.

OP’s posts: |
Cornettoninja Thu 19-Nov-20 09:50:03

My friend said thanks @SexTrainGlue.

@Taciturn - I understand what you’re saying about the context of this being from a court but it’s presentation (particularly to the public and uneducated masses like me) reads like it had been unequivocally found that PCR tests have a 97% false positive rate when in fact that’s the assertion of one report presented in a court case that from the sounds of it hasn’t been published or peer reviewed? (Happy to be corrected btw).

With so much misinformation flying around at the moment that’s a fairly dangerous title which doesn’t really represent the facts of what it’s communicating at face value.

PrivateD00r Thu 19-Nov-20 10:02:32

QueenStromba, that is very interesting, thank you!

DisorganisedPurpose Thu 19-Nov-20 10:06:17

It sounds like the test is really erring on the side of caution. But even if there is only. A tiny amount of viral load found (e.g. At 35 cycles), could it not be the case that over thr next days that load in tested person could multiply. I don't know how the biology of the virus works but if it could be multiplying in the body perhaps that's why the test has a low threshold for positive.

LizzieAnt Thu 19-Nov-20 10:11:17

With so much misinformation flying around at the moment that’s a fairly dangerous title which doesn’t really represent the facts of what it’s communicating at face value.

I agree.
You've made this about the science by choosing the title and quotation that you did OP.

FeelingBIue Thu 19-Nov-20 13:02:09

Taciturn

*It is staggering how science is so misreported.*
It's bollocks

You are quite mistaken @SexTrainGlue

It is a report about a portguese appeals court - with a quote directly attributed to that court - concerning PCR testing in Portugal.
It is NOT a science report at all, let alone a misreporting of one

You can claim that the court has the science wrong.
You could also claim that the report is fabricated (although I did try to verify this before posting), or that the quote is mis-translated or mis-attributed.

But at this stage it is a finding of fact that a Portuguese court made this claim. Incidentally, the ruling in favour of the plaintive was not depending on the finding.

Did you actually read the articles by Covid-sceptics you linked to Taciturn and the scientific papers quoted or did you go straight for click-bait headline for your 15 minutes of MN fame?

The appeal failed primarily because the court said that only a physician has authority to declare someone ill or a health hazard. The PCR test in question wasn't carried out by a physician, nor did a physician examine that person.

Therefore the court concluded that there was no evidence.

Actually a very interesting ruling if it follows that quarantine/isolation can't be legally enforced in Portugal unless a positive PCR test was carried out by a physician.

The mention of the two scientific papers in the ruling is interesting because it deals with the viral load and the cut-off point (CT-35) of the PCR tests. The court concluded from the papers that at a CT threshold of 35 or higher (i.e. CT 35 or over) then the test must be assessed in context of probability of disease.

It did not mention anywhere that positive PCR tests at CT-35 were false and it certainly did NOT rule that 97% of PCT tests are false.

The only false thing here is your ENTIRELY FALSE headline.

Did you read the scientific papers or maybe you just didn't understand them. So perhaps an analogy regarding CT cycles might help.

I have 35 buckets. I put one cup of sugar into each container. In the first bucket I add one cup of water. In the second bucket I add 2 cups of water. In bucket three I add 3 cups of water .... and so on. By the time I get to bucket 35 it contains the one cup of sugar and 35 cups of water.

I get 100 people to taste the water from each bucket.

Bucket 25 - 70% of people can still taste sugar.
Bucket 30 - 20% can still taste sugar.
Bucket 35 - <3% can still taste sugar.

Based on those findings, YOUR headline would read "97% of buckets give false positive for sugar"

See the problem with that statement?

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