Rapid antigen tests show reliability in close contacts of COVID-19 patients

Two rapid antigen tests demonstrate more than 60-percent sensitivity in asymptomatic and presymptomatic people tested on day 5 onwards following close contact with individuals with confirmed SARS-CoV-2 infection, reports a study. This sensitivity has increased to more than 85 percent after applying a viral load cutoff as proxy of infectiousness.

“The Veritor [System] and Biosensor rapid antigen tests can therefore reliably be used to test close contacts for infectiousness from the fifth day after infection, even when they have not (yet) developed symptoms, but the tests should not be used when the consequences of missed infections will be severe,” the researchers said.

A prospective cross-sectional study was performed in four public health service COVID-19 test sites in the Netherlands, including 4,274 close contacts aged ≥16 years and asymptomatic for COVID-19 when requesting a test. Participants were identified through test-and-trace programme or contact tracing app.

Some 233 (8.7 percent) of 2,678 participants tested with Veritor had RT-PCR confirmed SARS-CoV-2 infection; 149 of these were also detected by the rapid antigen test (sensitivity, 63.9 percent, 95 percent confidence interval [CI], 57.4–70.1). With Biosensor, 132 (8.3 percent) of 1,596 participants tested had RT-PCR confirmed SARS-CoV-2 infection, of whom 83 were detected by the rapid antigen test (sensitivity, 62.9 percent, 95 percent CI, 54.0–71.1). [BMJ 2021;374:n1676]

Among asymptomatic participants at the time of sampling, Veritor (n=2,317) showed a sensitivity of 58.7 percent (95 percent CI, 51.1–66.0) and Biosensor (n=1,414) 59.4 percent (95 percent CI, 49.2–69.1). In symptomatic patients, sensitivities were 84.2 percent (95 percent CI, 68.7–94.0) and 73.3 percent (95 percent CI, 54.1–87.7) for Veritor (n=219) and Biosensor (n=158), respectively.

After applying a viral load cutoff for infectiousness (≥5.2 log10 SARS-CoV-2 E gene copies/mL), the overall sensitivity was 90.1 percent (95 percent CI, 84.2–94.4) for Veritor and 86.8 percent (95 percent CI, 78.1–93.0) for Biosensor. Among those who remained asymptomatic throughout, sensitivity was 88.1 percent (95 percent CI, 80.5–93.5) for Veritor and 85.1 percent (95 percent CI, 74.3–92.6) for Biosensor.

In all analyses, both rapid antigen tests demonstrated >99-percent specificity, >90-percent positive, and >95-percent negative predictive values.

“The sensitivities of the tests were expectedly lower than those generally reported for people with symptoms and higher than those for asymptomatic people who are tested at random,” the researchers said. [https://lci.rivm.nl/antigeensneltesten]

The sensitivity of both rapid antigen tests was close to 90 percent in asymptomatic participants who developed symptoms between test request and actual testing, which is similar to previous studies of symptomatic patients. For those who did not develop symptoms, the sensitivity was close to 60 percent, which is greater than that reported in earlier studies. [https://lci.rivm.nl/antigeensneltesten; Cochrane Database Syst Rev 2021;3:CD013705.pmid:33760236]

“However, those studies generally focused on those who were tested at random and not because they had been exposed to an index case (close contacts),” the researchers said.

“As a result of [the current] study, early in 2021 Dutch test sites implemented the two (and other nationally approved) rapid antigen tests for testing of close contacts, even when they have not (yet) developed symptoms,” they noted.

Compared with RT-PCR testing, rapid antigen testing reduces delays, simplifies logistics and lowers dependence on equipment, allowing for tests in the community and for self-testing. The extent to which these benefits outweigh the lower sensitivity relative to RT-PCR is unknown, the researchers said.