Discussion
infection
RECRUITMENT, VIRUS, SARS-COV-2 INFECTION, DISEASE, RECRUITMENT, INFECTION
The results of this prospective diagnostic accuracy evaluation of two POCTs for the detection of SARS-CoV-2 in symptomatic patients in primary care fall within the wide range of previous studies in other settings [The minimum target for acceptable performance in the target product profile of the Medicines and Healthcare products Regulatory Agency, is sensitivity of 80% and specificity of 95% [Diagnostic performance was strongly associated with RT-PCR cycle threshold. Performance declined at higher cycle thresholds, which are associated with the presence of lower intact sample viral RNA, a proxy for viral load. Test sensitivity declined among individuals whose symptoms began more than one week before recruitment. Correlation has been proposed between higher viral load distributions, LFD positive results and infectiousness of individuals [Venekamp et al’s community-based study in the Netherlands of tests including SD Biosensor and BD Veritor recruited until June 2021, before the Delta variant became dominant [Other studies have shown substantial decreases in test sensitivity in asymptomatic individuals, including those recruited as close contacts of cases [This study prospectively recruited a large cohort of symptomatic participants attending primary healthcare and two COVID-19 testing centres, and therefore reflects real-world diagnostic accuracy. Understanding performance in primary healthcare is likely to be increasingly important as we cope with waves of endemic infection, and this is one of the few studies to report performance for any POCT in this setting and to our knowledge the only one based in UK primary care.Recruitment met recommended sample sizes. Further, this study benefitted from contemporaneous swabbing for all tests, and blinding of index test results from those who were performing the reference test (and vice versa), as recommended in diagnostic accuracy studies [This study also has some limitations. Because of low recruitment from some sites, a testing centre was added as a recruitment site and so the population tested overall may be less unwell than those who would contact the GP surgery. Prevalence of SARS-CoV-2 infection varied substantially by practice, suggesting there may have been differences in the way in which practices identified participants for recruitment. However, throughout the study recruited patients were required to be symptomatic and diagnostic performance did not change when restricted to participants recruited via the testing centre. This study does not assess diagnostic performance in asymptomatic patients, in whom viral load may be lower and there may be a consequent effect on diagnostic performance. It assesses performance when testing was carried out by clinical staff, rather than via self-swabbing, and performance might decline if not always done according to manufacturers’ instructions and performed by a trained operator. Consistent with other studies, we have used RT-PCR cycle threshold as a proxy for viral load and did not apply a calibration and conversion to provide absolute estimates. Fully quantitative assays require a calibrated standard curve, which was not incorporated as an element of this study, as the results were intended to be binary in recognition of how diagnostic decisions are made in the real world.This study represents 12 months of recruitment, during which time the prevalence of SARS-CoV-2 fluctuated, and results cannot necessarily be extrapolated to future variants should they emerge. For example, some studies have suggested that some assays may have impaired detection for Omicron variants [The number of missing test results was higher than anticipated, and RT-PCR results were unobtainable for 40 samples, most of which were from participants who received the SD Biosensor POCT during the early period of recruitment. The effect of this was explored in sensitivity analyses, which did not show substantial changes in the major results. The principal reason for missing RT-PCR data was because of postal delays during the pandemic period in the early set-up of the study. As such we consider these data to be missing completely at random and do not expect this to bias the results.In a population with symptoms of COVID-19 presenting to community settings, SD Biosensor and BD Veritor POCTs performed by healthcare professionals are highly specific and so could be used to rule in COVID-19. However the proportion of patients with positive RT-PCR test results who received false negative POCT results was 16.0% for SD Biosensor and 23.5% for BD Veritor, which could result in onward transmission and inappropriate management unless population prevalence of disease is very low. Performance was improved in patients with more symptoms and those with low RT-PCR Ct values. Tests should be interpreted with more caution outside of this clinical phenotype. Though this strategy was not tested, it may be sensible to repeat the POCT in 12 or 24 hours in patients with a clinical phenotype for COVID-19 who test negative since viral counts may rise over time. This strategy should be studied since identifying true negatives as well as positives is important as waves of this virus continue.