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Lateral flow antigen tests (LFATs), also known as rapid antigen tests, use a device with an absorbent pad at one end and a reading window at the other to detect SARS-CoV-2 antigen in nasal swab samples. They have become preferred worldwide for detecting SARS-CoV-2 infections because of their low cost and rapidly available results, despite limited sensitivity. Nonetheless, the likelihood of infectious COVID-19 cases being missed by false-negative LFAT results has not been closely evaluated. U.K. researchers reanalyzed and linked the data from four studies of LFAT accuracy in relation to viral load as indicated by RT-PCR assays, viral culture, and contact tracing. Settings included a test-and-trace center (70 symptomatic individuals), mass testing (111 asymptomatic individuals), and a university screen (62 asymptomatic students).
The Innova LFAT (primarily used in the U.K.) would miss a predicted 20% of symptomatic cases at the test-and-trace center who had positive viral cultures, 29% of asymptomatic individuals in mass testing, and 81% of students. Of the sources of secondary cases, 38%, 47%, and 90% would be missed, respectively. Further modeling suggested that to attain 97% sensitivity for probable infectious cases, LFATs should be able to detect viral loads 1000–10,000 times lower than the current limit of detection — unachievable with the currently available technology.
Deeks JJ et al. SARS-CoV-2 antigen lateral flow tests for detecting infectious people: Linked data analysis. BMJ 2022 Feb 23; 376:e066871. (https://doi.org/10.1136/bmj-2021-066871)
Comment
This complex recompilation and analysis of published data suggests that at least the Innova LFAT probably misses substantial proportions of infectious SARS-CoV-2–infected individuals, especially those who are asymptomatic. Whether the findings apply to all LFATs is not clear, but they certainly challenge the claim that such tests “detect the vast majority of infectious COVID-19 cases.”