Knowledge
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Early in the COVID pandemic, diagnostic testing relied entirely on precise-but-expensive PCR testing. Late in 2020 the Lateral Flow Testing techniques, already widely used for home-pregnancy tests and similar, were developed for SARS-CoV-2 antigens, leading to COVID-19 Rapid Antigen Tests (RATs).
While cheap, scalable and able to give a result in 10-15 minutes, they were initially seen mostly as a supplement to PCR testing, with less accuracy. Although true that RATs have lower sensitivity than SARS-CoV-2 PCR – most licensed-RATs have sensitivity 80-95% – today this is both less important, and possibly even a strength of RATs over PCR.
Early in the pandemic the role of testing has primarily about diagnosis, in those either symptomatic or pre-symptomatic. Viral presence was practically assumed to be synonymous with contagion. Today with over half a billion cumulative COVID cases worldwide and counting, along with access to effective vaccines and antivirals, it is often more useful to know whether an individual is infectious or not at a discrete moment in time.
Growing research over the last 12 months shows that adequately-sensitive RATs are effective at identifying infectious individuals, even if the high-sensitivity of PCR testing identifies viral particles in those who are infected but otherwise non-infectious (either pre-infectious, or post-infectious with ongoing viral shedding).
PCR positive results with cycle thresholds (ie. number of thermal cycles of RNA replication required before fluorescence is detected) above 25-30 have good correlation with being non-infectious (ie. unable to culture virus). Adequately approved & validated RATs (by FDA, TGA, MHRA, etc.) have very high sensitivity at CT less than this 25-30 range, depending on the study and specific manufacturer.
The bottom line...
An adequately-validated RAT, when correctly performed, is likely a sensitive indictor of individual infectiousness at that specific moment in time. The reliability of a negative RAT will be improved if using the same manufacturer and technique as a RAT previously positive test, and more so if there are several subsequent negative RATs.
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