Hi.
Forgive me if this subject has been raised elsewhere. What are users views on efficacy of C-19 testing?
I work in a care home for the elderly and we are under huge pressure to be tested, either by nurses who visit to test staff or using self testing kits. Test involves taking swab from nostril and throat.
I am sceptical about reliability of test results and have no faith in U.K. Gov handling of whole pandemic issue. I have refused test so far but today I was interviewed by my manager and asked what my reasons were for my refusal to test. He is under enormous pressure from his manager to have all staff tested so was sympathetic to my feelings but also managed to hint that action would be taken if I continued to refuse a test. Views please.
I work on Scotland, I am not a carer but a cleaner employed by local authority. My union is useless. It was hinted that management would ‘risk assess’ my situation and probably remove me from my workplace and place me somewhere I can do less harm!
The RT-PCR test works by looking for little bits of viral RNA and then amplifying them.
The number of "cycles" of amplification you do allows you to tune the test depending on whether sensitivity or specificity is more important.
Sensitivity is how good the test is at finding the thing you're looking for. Set the cycle threshold too low and you might miss cases, especially if you didn't get a decent swab.
Specificity is how good the test is at not giving you false positives. Crank the cycle threshold right up and the test will start detecting its own primers and report that it has found SARS2 RNA In a pineapple.
The UK tests will be using a reasonable cycle threshold (or were the last time I checked) giving them a sensitivity of about 70% and a specificity of at least 95% probably a bit better. So this means if you test 100 people who are infected it will miss about 30 if them. But if you test 100 people who are not infected it will falsely tell you about 5 of them are. It can also find smashed up bits of virus in people who have recovered so this will reduce the specificity a bit more (but I don't know by exactly how much, it's fairly academic anyway, see below).
The specificity of the test becomes dramatically more important the lower the actual prevalence of the thing you're looking for. Because levels of Covid in the UK are now so low, if you test positive it doesn't mean there is a 95% you actually have Covid. If 5 out of 100 tests are false positives but only 0.03% of people really have Covid (which is about the current UK prevalence) then there is only about a 0.6% chance that having tested positive you really have Covid (0.03/5).
If you test negative it also means you probably don't have Covid because even though 30% people will test negative anyway if they have Covid, you only had a 0.03% of having Covid anyway.
So in conclusion the test is a perfectly good test with a reasonable sensitivity and specificity. At the height of the epidemic if you'd had one that gave you results quickly it would have been useful. But now that the prevalence in the UK is so low, the test is not able to give a meaningful result to an individual just because of the math. The test result just tells you whether it is exceedingly unlikely you have Covid or extremely unlikely you have Covid.
It only has a use now when doing population level studies with very large samples. You need to test tens of thousands of people at random to try to gauge the real prevalence.
I should mention another source of false positives is the test finding not SARS2 but a different related coronavirus. Scientifically speaking it ought to be reasonable to require a match on sequences that are unique to SARS2 (at least out of known coronaviruses) but there have been reports of the WHO playing silly buggers with the official test criteria. I don't know how significant that is.
I should mention another source of false positives is the test finding not SARS2 but a different related coronavirus. Scientifically speaking it ought to be reasonable to require a match on sequences that are unique to SARS2 (at least out of known coronaviruses) but there have been reports of the WHO playing silly buggers with the official test criteria. I don't know how significant that is.
I have heard coronavirus are a family of viruses and I was also wondering if people testing positive have another coronavirus.
