The mass testing of asymptomatic folks for COVID-19 within the UK was thrown into query by a current research. In a pilot in Liverpool, over half the circumstances weren’t picked up, main some to query whether or not utilizing checks that carry out poorly is the very best use of assets.

The checks concerned on this research had been antigen checks. These see whether or not somebody is contaminated with SARS-CoV-2 by figuring out buildings on the surface of the virus, often 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 checks are low-cost and supply outcomes rapidly. Nonetheless, they don’t seem to be all the time correct. However what will we imply after 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 all for 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 probably it’s to ship a false unfavorable outcome to somebody who has the virus.

False negatives can have vital prices. If folks receiving them are additionally infectious, this will improve the danger of viral transmission, as they’ll behave as in the event that they don’t have the virus – what’s often known as “false reassurance”.

However sensitivity just isn’t the one sort of accuracy that issues – we’re additionally all for how good the check is at offering optimistic outcomes solely to those that are literally contaminated. The extra particular a check is, the much less probably it’s to ship false positives to these with out the virus. False positives even have prices – an individual’s liberty is perhaps restricted although 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 unfavorable price, risking widespread false reassurance. The check can not robustly affirm that somebody isn’t contaminated.

Nonetheless, there are different related factors to think about from the Liverpool pilot. First, the research discovered that the specificity of the check was 99.93%. That signifies that solely a small proportion of contributors who weren’t contaminated got a optimistic outcome by the check. This specificity is an efficient factor, however we shouldn’t overstate its significance; excessive specificity alone doesn’t entail {that a} optimistic result’s more likely to be a real optimistic. This probability, or the check’s “optimistic predictive worth”, can also 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 optimistic outcome to 0.07% of them – roughly 70 folks. So on this situation, assuming the check is completely delicate and picks up all of the true positives, there would solely be a 50% likelihood of a optimistic outcome being true.

Curiously, the Liverpool knowledge additionally suggests that almost all of true optimistic 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 can be a very powerful asymptomatic circumstances to determine.

The upshot of that is that antigen testing has some options in its favour for figuring out optimistic circumstances. The issue is that these advantages could also be small if the virus just isn’t prevalent, they usually 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 danger of false reassurance, nevertheless it could possibly be modified. The marketed goal of antigen testing might as a substitute be to determine extra of the asymptomatic carriers at the moment flying below 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 signifies that many asymptomatic carriers are being missed, and that’s an issue – roughly 40-45% of infections are estimated to be asymptomatic.

It may additionally be attainable to make clear to those that optimistic outcomes are strong in a method that the unfavorable outcomes usually are not. We might additionally impose additional restrictions on folks with optimistic outcomes with out equally utilizing unfavorable outcomes as justification for releasing people from different current restrictions.

One drawback with all of those methods is that they’re troublesome public well being messages to speak. Nonetheless, the extent of the issue of false reassurance can also be decided by the proportion of infectious folks among the many false unfavorable circumstances. The Liverpool knowledge suggests an avenue of additional research right here.

If we might 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 checks 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 this stuff. Nonetheless, if we might, then it might help the argument that these checks might 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 a variety of different questions stay. But it surely’s nonetheless attainable that some type of mass antigen testing might but be helpful sooner or later.

Jonathan Pugh at the moment receives funding from the UKRI, and has beforehand acquired funding from the Wellcome Belief