Just how effective is mass PCR testing?

Amid the government’s statements about mass testing programmes throughout the coronavirus crisis, reservations have often been expressed about the efficacy of PCR tests. These are the types of tests on which the majority of the UK’s testing efforts have relied. Where advocates for the large-scale testing of the population argue that it is an essential method for getting a grip on Britain’s epidemic, critics often point to shortcomings in the nature of the tests themselves, including the possibility of false positive and negative results.

So who is right? Well, the debate isn’t always quite as binary as it seems. Scientists who are proponents of mass testing tend to acknowledge what testing can and cannot do while those who are sceptical about mass testing programmes often value a more targeted approach to testing rather than no testing at all. This targeted testing would in practice mean prioritising specific locations such as hospitals and care homes over the testing of the healthy population in the community.

The PCR test is widely held to be the most reliable form of test for the virus, with the best sensitivity and possibly also specificity of the available tests. Yet it is not perfect – no method of testing is – and it is important to note that PCR test results still require much interpretation. Notably, it is very sensitive, but it performs less well on specificity – it is better at picking up a true positive than a true negative.

It is worth reminding ourselves of how PCR tests are carried out. They detect specific genetic material within the SARS-CoV-2 virus called RNA. A sample swab is taken from the lower respiratory tract and from the nasal passage. This swab is then either sent off to a laboratory where it is processed, or it can now also be processed remotely, using DNANudge machines.

The process that produces the result is a reverse transcriptase polymerase chain reaction (hence RT PCR). The reaction used in the test in turn indicates whether viral RNA is present in the sample.

What this means is that a PCR swab is just one sample from a single moment in time – not a cast-iron guarantee that someone is free from infection.

For instance, an article by the Centre for Evidence-Based Medicine at the University of Oxford emphasises that while a PCR test “is very sensitive and will detect the presence of viral RNA”, it is not always clear whether this viral RNA is active and has the capacity to reproduce and cause infections.

On top of this, a systematic overview of the effectiveness of testing in the British Medical Journal published on September 30 has concluded that “Evidence is rapidly emerging on the effectiveness of tests for Covid-19 diagnosis and management, but important uncertainties and their effectiveness and most appropriate application remain.”

Then there is the particular problem of false negatives that critics of mass testing often point towards. This is where a test result is shown to be negative but has in fact failed to pick up that someone is infected by the SARS-CoV-2 virus and is infectious. One journal article from May 2020 suggested that the rate of false negatives for PCR tests could be as low as 2% or as high as 37%.

Dr Joshua Moon, research fellow in the Science Research Policy Unit at the University of Sussex, has summarised this problem well: “A negative result could be that the individual is truly negative and therefore not infectious, or it could be that the individual is infected but early in the incubation period so isn’t testing positive yet, or it could be that the test itself didn’t capture enough viral material on the swab or saliva to test positive. In only one of these cases should the person be moving around as normal.”

He adds to this technical problem a logistical challenge to consider: “On top of this, mass testing can and likely will result in higher numbers of cases being found. This means two interrelated problems: first, more cases means more contacts which means more burden on an already over-stretched tracing system.”

In light of such difficulties, it has sometimes been posited by some critics that the mass testing of asymptomatic and otherwise healthy people is a vast waste of resources and effort that could be better spent on other things.

Yet this is to touch precisely on the central conundrum of the Test and Trace programme – it can equally be argued that, if mass testing is to be a success, it does need to test otherwise asymptomatic people on a large scale, and regularly. Community transmission of the virus, outside of hospitals, is most likely to come from those who are asymptomatic. If only a significant number of these asymptomatic cases were to be picked up when they would otherwise go undetected, then even this could still make a dent in the overall prevalence and spread of the virus.

To risk repeating a well-worn Covid cliché, PCR testing is not a “magic bullet” – but, yet again, neither is a vaccine. Nonetheless, scientific experts, whatever their disagreements about the government’s current Covid strategy, recognise that both of these tools have a part to play in studying, understanding, and combatting Covid-19.

There should always be room for rational debate about the potential utility of testing as well as its shortcomings – just as there ought to be a mature discussion about lockdowns, Covid tiers, and mask-wearing.  But this debate must also be fundamentally grounded in a balanced awareness of the available evidence, not conspiracy theories or political fantasies.