INTRODUCTION
The control of coronavirus 2019 disease (COVID-19) relies on implementing an effective testing and tracing strategy; the ideal would be a move to using rapid point-of-care tests (POCTs) in the community, to detect both active infections and to identify those already immune, as being tested in Liverpool.1 To date, largely laboratory-based virology testing (reverse transcription polymerase chain reaction (PCR) to detect viral ribonucleic acid from naso/oropharyngeal swabs) remains the gold standard for diagnosis. The role of COVID-19 antibody testing is limited, with few studies published about the diagnostic accuracy of POCTs that might be used in primary care. This article aims to summarise the current state of knowledge about the potential role of COVID-19 antibody testing in primary care.
THE ROLE OF ANTIBODY TESTING IN COVID-19 INFECTION
Following infection, we develop immunoglobulins (Ig) against SARS-CoV-2 proteins, including the diagnostically important nucleoprotein and/or spike proteins (N- and S-proteins, respectively). The median time of detection of anti- SARS-CoV-2 IgG is day 14 post-onset of symptoms (POS) (for IgM and IgA it is day 5), and it is detectable in 90% of individuals by then.2,3 Anti-N-protein immunoglobulin class G (IgG) is the most sensitive current target for serological detection, albeit its combined detection with anti-S improves sensitivity.4–6 IgA antibodies, implicated in mucosal surface defence, are detectable from the first day POS.3
A waning in IgG response has been reported,7 with potentially severe implications for long-term immunity depending upon the alternative effectiveness of memory T-cell-mediated immunity. There are suggestions that seropositive individuals are only at a lower risk of re-infection compared with seronegative individuals.8 There is no definitive evidence present about the protective effect, if any, of anti-SARS-CoV-2 antibody against reinfection and/or COVID-19 disease. Individuals previously infected with COVID-19 can serorevert and some who have seroconverted do not necessarily stop shedding the …
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