The mass testing of asymptomatic folks for COVID-19 within the UK was thrown into query by a current examine. In a pilot in Liverpool, over half the circumstances weren’t picked up, main some to query whether or not utilizing assessments that carry out poorly is the very best use of sources.
The assessments concerned on this examine had been antigen assessments. These see whether or not somebody is contaminated with SARS-CoV-2 by figuring out buildings on the skin of the virus, generally known as antigens, utilizing antibodies. If the coronavirus is current in a pattern, the antibodies within the check bind with the virus’s antigens and spotlight an an infection.
Antigen assessments are low cost and supply outcomes shortly. Nonetheless, they aren’t all the time correct. However what can we imply once we say {that a} check is inaccurate? And is it actually the case that “an unreliable check is worse than no check”?
Sensitivity vs specificity
When testing, one factor we’re taken with is how good a check is at detecting the virus in people who find themselves really contaminated. The extra delicate a check is, the much less doubtless it’s to ship a false adverse consequence to somebody who has the virus.
False negatives can have vital prices. If folks receiving them are additionally infectious, this may increasingly enhance the chance of viral transmission, as they’ll behave as in the event that they don’t have the virus – what’s generally known as “false reassurance”.
However sensitivity shouldn’t be the one type of accuracy that issues – we’re additionally taken with how good the check is at offering constructive outcomes solely to those that are literally contaminated. The extra particular a check is, the much less doubtless it’s to ship false positives to these with out the virus. False positives even have prices – an individual’s liberty is perhaps restricted despite the fact that they pose no danger of transmission.
The Liverpool knowledge
Preliminary knowledge from the Liverpool pilot means that the check used was 48.89% delicate. That interprets into a really excessive false adverse price, risking widespread false reassurance. The check can’t robustly affirm that somebody isn’t contaminated.
The British Military helped to ship the mass testing in Liverpool, which was launched in early November.
EPA-EFE
Nonetheless, there are different related factors to think about from the Liverpool pilot. First, the examine discovered that the specificity of the check was 99.93%. That implies that solely a small proportion of members who weren’t contaminated got a constructive consequence by the check. This specificity is an efficient factor, however we shouldn’t overstate its significance; excessive specificity alone doesn’t entail {that a} constructive result’s prone to be a real constructive. This probability, or the check’s “constructive predictive worth”, can be partly decided by how prevalent the virus is within the examined inhabitants.
As an illustration, say you check 100,000 folks with a check that’s 99.93% particular, but the speed of COVID-19 on this group is comparatively low – solely 70 circumstances per 100,000 folks. Among the many 99,930 people who find themselves uninfected, the check would nonetheless return a false constructive consequence to 0.07% of them – roughly 70 folks. So on this state of affairs, assuming the check is completely delicate and picks up all of the true positives, there would solely be a 50% likelihood of a constructive consequence being true.
Curiously, the Liverpool knowledge additionally suggests that almost all of true constructive outcomes had been in people who had greater viral masses. If – and it’s an if – greater viral masses are strongly related to higher infectivity, then these might be an important asymptomatic circumstances to determine.
The upshot of that is that antigen testing has some options in its favour for figuring out constructive circumstances. The issue is that these advantages could also be small if the virus shouldn’t be prevalent, and so they could also be massively outweighed by the prices of false reassurance whether it is widespread.
Can we keep away from false reassurance?
There is perhaps some measures that would probably scale back these prices. The present messaging that elevated testing can “present reassurance” amplifies the chance of false reassurance, nevertheless it might be modified. The marketed objective of antigen testing may as an alternative be to determine extra of the asymptomatic carriers at the moment flying beneath the radar.
Some context is necessary right here. Within the UK, extra correct testing is at the moment freely accessible just for symptomatic people and a small variety of different teams. This technique implies that many asymptomatic carriers are being missed, and that’s an issue – roughly 40-45% of infections are estimated to be asymptomatic.
If authorities clarified that check outcomes are usually not black and white, then folks’s behaviour after being examined is perhaps totally different.
EPA-EFE
It may additionally be potential to make clear to those who constructive outcomes are sturdy in a means that the adverse outcomes are usually not. We may additionally impose additional restrictions on folks with constructive outcomes with out equally utilizing adverse outcomes as justification for releasing people from different present restrictions.
One downside with all of those methods is that they’re tough public well being messages to speak. Nonetheless, the extent of the issue of false reassurance can be decided by the proportion of infectious folks among the many false adverse circumstances. The Liverpool knowledge suggests an avenue of additional examine right here.
If we may set up firstly that individuals with low viral masses pose an acceptably low danger of transmission, and secondly that the false negatives generated by antigen assessments had been restricted to people with such low viral masses, then the hurt of those false negatives would even be low. We at the moment lack essential knowledge to definitively set up these items. Nonetheless, if we may, then it could help the argument that these assessments may nonetheless be used as an efficient containment technique, based mostly round extremely frequent testing.
There are vital challenges for mitigating the harms of inaccurate mass antigen testing, and numerous different questions stay. But it surely’s nonetheless potential that some type of mass antigen testing may but be helpful sooner or later.
