WHO notice on PCR tests

[GrinchTastic]

The WHO have issued new guidance on PCR tests, urging caution in how they are interpreted. It seems that they are saying that they shouldn’t be used to diagnose a case in the absence of symptoms.

Can anyone shed any light on this? I’ve seen chatter on Twitter to the effect that this will case case numbers to plummet (and some scepticism that this was issued on the day of Biden’s inauguration ) but I would love to hear informed, scientific perspectives on what this means.

Will this mean that some asymptomatic cases will no longer count as cases if the viral load is very small? And if the PCR test is problematic at the higher ends of the cycles, what does this mean for the reliability of the Lateral Flow tests?

There has always been this doubt. PCR tests are very inaccurate but being used as the main way to justify restrictions.

Check out Dolores Cahill on this.

This isn't new. It's been around for weeks. It was issued as some countries had operators not applying adjustments for noise which has always been an issue at higher cycle runs. Which means they shouldn't be reading the tests at all. It's a know analytical adjustment for any PCR test.

PCR is used to diagnose HIV and to assess disease progress. Any papers on false positives for HIV?? Or Hepatitis C?

So are these adjustments already factored into U.K. testing and current numbers Terryscombover?

Dolores Cahill of the (fascist) Irish Freedom Party? No thanks

@Calledyoulastnightfromglasgow

There has always been this doubt. PCR tests are very inaccurate but being used as the main way to justify restrictions.

Check out Dolores Cahill on this.

Nonsense. That's the last person anyone should be 'checking out' for reliable information, she was a speaker at the David Icke rally in London.

So I do know a little about this though can’t claim to be an Actual Scientist.

Essentially PCR testing is very sensitive (despite what a poster above wrongly claims) so eminently possible it will detect tiny viral loads indicative of an active infection many weeks previously, by which point one will be well beyond the point infectious. However, the results as issued at the moment (rightly) just give a straight you’ve got it or you haven’t.

In a different situation an actual dr would interpret the results, considering viral load, and factor that into a clinical decision about what action is required. Clearly that’s not an option here. So you’re either positive or negative. It means some people, especially if they don’t have symptoms might (stress MIGHT) be isolating past when they need to.

That said, one third of cases are asymptomatic so just because you don’t feel ill doesn’t mean you’re not infectious or that the PCR is somehow giving an inaccurate result.

PCR is used in a different way to lateral flow. The latter is primarily to detect asymptotic cases in people that wouldn’t otherwise get tested. I.e. it’s people who might have it (like I said, one third are asymptomatic) but won’t have any cause to get a test but may unwittingly but legitimately going about their business and spreading the virus. So they’re not perfect and don’t get every case - but every one they do detect is a case that wouldn’t get picked up otherwise.

@Terryscombover

PCR is used to diagnose HIV and to assess disease progress. Any papers on false positives for HIV?? Or Hepatitis C?

No test is 100% but if you want to compare: HIV tests - specifically blood drawn 4th generation tests that look for the antigen as well as antibodies - are one of the most accurate. More info here i-base.info/guides/testing/test-accuracy-results-and-further-testing

Much is said about inaccurate tests but I (up until recently) worked as a contact tracer. Out of the hundreds of people I spoke to probably no more than ten were asymptomatic. Yes its just anecdata I know but I remember at the time thinking where are all the asymptomatic people? Though its quite obvious that they probably weren't testing because they felt perfectly fine. The asymptomatic were usually care workers/medics who got picked up on the routine testing.

Actually what WHO have clarified that if the test result is weak and there are no symptoms, you must retest. Our labs are supposed to be retesting for weak results anyway.

And PCR tests are only for those with symptoms,

Lateral flow tests are used for mass testing.

Please don’t spread misinformation.

www.who.int/news/item/20-01-2021-who-information-notice-for-ivd-users-2020-05

That’s the WHO covering their arses as there has been huge exaggeration of the true figures

PCR tests have been used for testing close contacts/asymptomatic cases in other countries. That’s not misinformation, that’s a fact.

In my line of work I can think of several outbreaks in care settings involving asymptomatic staff spreading the virus. We don’t really have an alternative, do we? We need to consider them as true positives. In many cases people who were asymptomatic at time of testing have gone on to develop symptoms anyway.

PCR tests amplify genetic material. The widely acknowledged problem is, that if you ramp up the cycles of amplification enough, everyone tests positive, and if you don’t do enough cycles then no one tests positive.

Finding the sweet spot of amplification is difficult and sometimes unreliable. Even the inventor of the test warned that it really is not designed to accurately test for viral infection, it again we do not have a foolproof test.. So, the main issue is that it can be unreliable, but also it is probably the best we have at the moment.

It’s bloody difficult to test for a virus, and the blood antibody test won’t be accurate until the body has has time to generate enough antibodies in the blood, so no use for initial diagnosis.

@GrinchTastic

PCR tests have been used for testing close contacts/asymptomatic cases in other countries. That’s not misinformation, that’s a fact.

Your post is misinformation because you’ve misrepresented what the who said. They said retest weak results if there’s no symptoms not PCR tests are useless. Most labs do retest in these instances anyway.

There's some information on both tests here

I was under the impression the PCR test was unreliable, and also that we were using more amplification cycles than other countries.

I have not looked at research into lateral flow tests at all.

blogs.bmj.com/bmj/2020/12/08/peter-roderick-is-it-lawful-to-require-self-isolation-after-a-positive-covid-19-test/

I didn’t say the WHO said PCR tests were useless Turtleshelly. Not sure where you are getting that from

Dismissing as “misinformation” an attempt to ask a question about a complex process that has been subject to revision and updating is frankly bizarre.

Like I said, PCR tests have been used for asymptomatic close contact testing in other countries so if there’s anyone guilty of misinformation, it’s you.

[quote AcornAutumn]There's some information on both tests here

I was under the impression the PCR test was unreliable, and also that we were using more amplification cycles than other countries.

I have not looked at research into lateral flow tests at all.

blogs.bmj.com/bmj/2020/12/08/peter-roderick-is-it-lawful-to-require-self-isolation-after-a-positive-covid-19-test/[/quote]
I'm not aware that we are using more cycles. I believe the government guidance is to follow the manufacturer's recommendation. They also recommend retesting weak samples from what I remember, but no idea if that is happening.

Either way, the criteria for testing in the UK requires being symptomatic and we're hardly in the situation of cases being very low, so the chance of any individual positive result here being a false positive is probably negligible.

I believe the government guidance is to follow the manufacturer's recommendation.

Yes and that is all the WHO document is saying, it's just a technical document reminding labs to read the (manufacturers) instructions for use (IFU). They issued a similar reminder last year. UK labs would be doing this anyway.

Most positive results are