key: cord-0888311-l6ybm9cy
authors: Ladhani, Shamez N.; Chow, J. Yimmy; Atkin, Sara; Brown, Kevin E.; Ramsay, Mary E.; Randell, Paul; Sanderson, Frances; Junghans, Cornelia; Sendall, Kate; Downes, Rawlda; Sharp, David; Graham, Neil; Wingfield, David; Howard, Rob; McLaren, Robert; Lang, Nicola
title: REGULAR MASS SCREENING FOR SARS-CoV-2 INFECTION IN CARE HOMES ALREADY AFFECTED BY COVID-19 OUTBREAKS: implications of false positive test results
date: 2020-09-16
journal: J Infect
DOI: 10.1016/j.jinf.2020.09.008
sha: d799344e6e548de15be4ba9480d7fc043be3ed67
doc_id: 888311
cord_uid: l6ybm9cy

nan

Shamez N Ladhani, 1 J. Yimmy Chow, 1 Sara Atkin, 1 Kevin E. Brown, 1 Mary E. Ramsay, 1 Paul Randell, 2 Frances Sanderson, 2 Cornelia Junghans, 3 Kate Sendall, 2 Rawlda Downes, 2 3 leading to the national implementation of widespread screening of care home staff every week and of residents every 28 days to identify and isolate infected individuals and limit the spread of SARS-CoV-2 in care homes. 4 In July 2020, Public Health England (PHE) was informed of two asymptomatic staff and one asymptomatic resident in the same care home who tested positive for SARS-CoV-2 through national screening (Table) . This care home had been part of the initial outbreak investigation in April 2020 and all three individuals were known to have SARS-CoV-2 antibodies. 5 We, therefore, undertook additional investigations to assess whether these were re-infections or false positive results and discuss the implications of our findings for residents, staff and care homes in general.

Following our initial COVID-19 outbreak investigations in April 2020, 3 (Table) .

The care home immediately re-instituted lockdown procedures. Following the new positive SARS-CoV-2 RT-PCR result, all three individuals were re-tested at PHE national reference laboratory within 24 hours and were RT-PCR negative with detectable SARS-CoV-2 antibodies. Additionally, all residents and staff -including the three individuals -were re-tested for SARS-CoV-2 RNA as part of the outbreak management and were negative. Four weeks later, repeat testing in the two staff showed no rise in SARS-CoV-2 antibodies. The resident was also antibody positive 4 weeks later but the test was performed in a different laboratory which did not report quantitative results.

The protective role of SARS-CoV-2 antibodies against re-infection and disease remains to be established, but there is increasing evidence showing that those with neutralising antibodies are unlikely to be infected with live virus, 8 which in turn reduces their risk of infecting others. Despite the large numbers of ongoing COVID-19 outbreaks in England, 9 these four London care homes did not have any additional cases prior to the national screening programme. The reporting of three positive results in a single care home was, therefore, unexpected and prompted additional investigations, which included repeat swabs, which were all negative, and blood sampling which confirmed their seropositivity at the time of re-testing. The lack of an antibody rise four weeks later confirms that these detections were not new infections and, therefore, false positive screening tests.

Further work needs to be undertaken to assess the value of repeated mass swab testing in care homes during periods of low community prevalence, 10 particularly if SARS-CoV-2 positivity rates fall below 1%, when the likelihood of false positive results increases exponentially even with RT-PCR assays that have very high specificity rates (Figure) . This can have a significant impact on care homes, in terms of unnecessary isolation of vulnerable residents and loss of workforce leading to suboptimal care provision. 11 This problem of false positivity has recently been recognised, with new national guidance published on how to interpret low level RT-PCR positive samples, including a recommendation to retest all samples testing positive at the level of detection of the assay before undertaking wider public health action. 12 It is hoped that this recommendation will reduce the number of similar closures of care homes or other institutions exposed to mass testing as a result of non-reproducible positive SARS-CoV-2 RT-PCR results.

In conclusion, in care homes that have already experienced a COVID-19 outbreak, up to two-thirds of staff and surviving residents develop neutralising antibodies which is likely to reduce the risk of new infections and, particularly, further outbreaks. Whilst community SARS-CoV-2 prevalence is low, rather than repeated mass swabbing, there is an opportunity to assess a role for wider testing for SARS-CoV-2 antibodies to assess past exposure accompanied with early and rapid testing for SARS-CoV-2 RNA as needed. Any positive result could then initiate wider testing for SARS-CoV-2 RNA in the care home, include retesting the index case, and a more nuanced risk assessment of the likelihood of a true outbreak.

Funding: none 

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