Diagnostic tool or screening programme? Asymptomatic testing for SARS-CoV-2 needs clear goals and protocols.

Abstract

Countries’ increases in testing capacity during the first waves of the COVID-19 pandemic, coupled with reductions in case numbers between waves, have resulted in shifts from diagnostic testing of symptomatic patients to mass screening. Viewed from the perspective of United Kingdom testing, it is therefore arguable that the largest asymptomatic testing programme, by testing rate, ever attempted in the country is currently being deployed. Other countries are deploying similar programmes. Even with the advent of further waves of infection, especially associated with northern hemisphere winter, asymptomatic testing is still occurring, and it can be anticipated that as waves recede and seasons change it will again increase. These shifts have led to policies that risk conflating diagnosis with screening. Currently, there are local, regional and national variations in criteria for screening, in modes of delivery, in whether laboratories undertake confirmatory testing following positive screen results, in the extents of contact tracing undertaken, and in quality assurance of programmes, signifying discrepancies in the understanding and policy objectives of the screening being undertaken. Unavoidably, variations within the populations being tested, in terms of both disease prevalence (and hence the proportion of positive tests that are false positives) and the level of risk to the health of different individuals, also lead to implications for testing strategies, acceptability, and balancing the interests of individuals and society. Screening for SARS-CoV-2 is being undertaken with heterogeneous inclusion criteria and with heterogeneous aims [1]. The UK government's early policy stated a primary aim of making diagnoses in symptomatic individuals, with the main aim of the testing strategy being to send back to work high risk critical workers in whom a diagnosis of COVID-19 was not made [2]. However, this was rapidly extended to testing people without symptoms in care homes or returning from hospital to care homes [3,4]. More recently, there are reports of government plans to increase UK SARS-CoV-2 testing capacity to 10 million tests per day – sufficient to test the entire population each week – with these plans mentioning both symptomatic individuals and their contacts [5]. Individual organisations have used increased testing capacity to test people without symptoms with the aim of reducing transmission of SARS-CoV-2 [6,7]. Asymptomatic screening has been adopted in hospital settings to guide both infection control practices around those with higher risk of being infectious and timings of treatment for other conditions (such as elective surgery and cancer chemotherapy) where there may be a higher risk of adverse outcomes if infected [8], [9], [10], [11]. It has been adopted in community settings to allow rapid isolation and cohorting of infectious individuals in facilities and hence to reduce morbidity and mortality from institutional outbreaks [12], [13], [14]. It has even been used to monitor the progress and guide timing of containment measures for an entire town [15]. Wider screening of healthcare workers [16] and university students and staff [17] has been advocated. Testing strategies are being managed in a piecemeal fashion, but from a historical perspective this mirrors the introduction of many mass screening programmes. Heterogeneity within and between screening programmes for a single disease is not new [18,19], and as in historical cases when other screening was introduced in uncoordinated fashion, with the practice of screening ahead of evidence for its benefit, we now need to develop a systematic approach and ask to what ends we are screening, whether screening achieves these ends, and how we can approach screening methodically, in order that we can efficiently and economically achieve the best outcomes feasible as circumstances in the pandemic change. These are the purposes for which many countries have screening oversight organisations.