Nasal Self-Sampling Rapid Antigen COVID-19 Test Performance May Have Decreased Over Time

The performance of rapid antigen tests using nasal self-sampling decreased as time passed during the omicron period of COVID-19, according to investigators who recently analyzed the available data in a study published in BMJ.

“Sensitivities of 3 rapid antigen tests with nasal self-sampling decreased during the emergence of omicron,” wrote the authors of the study. “Sensitivities appeared to be substantially influenced by the proportion of confirmatory testers.”

Rapid antigen tests are effective tools to detect COVID-19 infection. They were originally created for trained professionals, but with minimal equipment and results available in 15 to 30 minutes, the tests have become a convenient over the counter option for the general population to detect the virus early and possibly reduce transmissions.

Researchers evaluated the diagnostic accuracy of 3 commercially available rapid antigen tests (Flowflex, MPBio, and Clinitest) with unsupervised self-sampling over time during the COVID-19 omicron period. Additionally, the researchers studied the effects of adding oropharyngeal to nasal self-sampling, using reverse transcription polymerase chain reaction (RT-PCR) testing as the reference.

The primary outcomes were diagnostic accuracy—including sensitivity—in all 3 tests (either with nasal sampling or combined oropharyngeal and nasal self-sampling), with RT-PCR as the control. Secondary outcomes included diagnostic accuracies based on reason for testing, COVID-19 vaccine status, previous COVID-19 infection, sex, and age.

During a period from December 21, 2021 to February 10, 2022, researchers identified 6497 participants from 3 public health service COVID-19 sites in the Netherlands who were aged 16 years and older and presented with COVID-19 symptoms. Participants were first swabbed for the RT-PCR control test. Then, the participants randomly received 1 of the 3 of the self-service rapid antigen tests. The participants then performed either nasal self-sampling or nasal self-sampling with oropharyngeal, depending on the time period.

Nasal self-sampling tests were given during phase 1 of the study, which was when omicron accounted for more than 90% of COVID-19 infections. Nasal self-sampling with oropharyngeal tests were given during phase 2, during which time omicron accounted for more than 99% of infections.

During phase 1 of the study, there was 79% overall sensitivity for Flowflex, 69.9% for MPBio, and 70.2% for Clinitest; however, sensitivity for Flowflex was found to be more likely to decrease over time. In phase 2, 53% of MPBio and 44.4% of Clinitest participants had confirmed positive SARS-CoV-2 cases on the self-test, with the overall sensitivities after combining oropharyngeal and nasal self-sampling being 83% and 77.3%, respectively. The team also observed that previously infected individuals, women, and people aged 40 years and older had lower sensitivities and a larger margin for error on their self-administered test.

The study was limited due to the sample size calculation, which was based on primary analysis. Additionally, the research team only estimated the weekly prevalence of omicron. Further, an additional limitation was that researchers used slightly different sampling methods for the reference test, which could produce bias for the results of the study.

“Manufacturers of rapid antigen tests should consider extending their instructions for use to include combined oropharyngeal and nasal self-sampling,” wrote researchers in the study.

Reference

Diagnostic accuracy of covid-19 rapid antigen tests with unsupervised self-sampling in people with symptoms in the omicron period: cross sectional study. September 14, 2022. BMJ. 2022;378: e071215. doi: 10.1136/bmj-2022-071215