By: Melanie Smith R.N. BScN, Senior COVID Testing Implementation Manager, COVIDdetect, by DriverCheck
Contributor: Dr. Jonathan Davids, Corporate Chief Medical Director, COVIDdetect, by DriverCheck
February 24, 2022
The Omicron variant (B.1.1.529) is driving a need for reassessment of testing strategies using rapid antigen tests (RATs) due to its impact on their performance. The Ontario COVID-19 Science Advisory Table’s (“Advisory Table”) briefing from February 10th, 2022, announced a number of updates and recommendations with respect to rapid antigen testing in light of recent studies on their effectiveness in relation to Omicron. Omicron remains the predominant strain circulating across Canada as well as internationally due its high rate of transmission and ability to cause re-infections and breakthrough infections in those who are fully vaccinated.
Combined sampling method recommended to increase reliability and sensitivity of RATs
As a means to increase the reliability and sensitivity of rapid antigen tests, the Advisory Table’s briefing recommends the adoption of the combined oral-nasal sampling method. Sensitivity of a rapid antigen test is its ability to accurately identify the presence of COVID-19. Swabbing, or sampling from, the anterior nares (nostrils) has been a popular means of sample collection as it is less invasive than others such as from the nasopharyngeal or oropharyngeal areas. One study noted in the briefing found the sensitivity of nasal-only samples to be 68.4% whereas combined throat-nasal samples had a sensitivity of 81.6%1.
The combined oral-nasal sampling method involves swabbing the inside of both cheeks plus the very back of the tongue or throat followed by both nostrils. Using this combined method increases the sensitivity of rapid antigen tests to be comparable to the sensitivity of nasopharyngeal samples which are considered the gold standard for testing and are a restricted method of sample collection that is only to be conducted by a trained, regulated health care professional.
As the combined oral-nasal sample collection method is more invasive and slightly more time consuming, it may be viewed as less desirable for voluntary, asymptomatic testing programs. Clear and evidence-based communication is key to encouraging the adoption of the combined method of sample collection. Organizations will need to critically evaluate and weigh the pros and cons of the combined method – if a less invasive, but still acceptable, option provides greater participation in the program, that may sway the decision away from adoption.
Serial testing: 3-5 times a week is recommended for asymptomatic surveillance testing
The studies reviewed in the Advisory Table briefing also found that individuals with Omicron have been found to be infectious days before testing positive. Due to this finding and together with rapid antigen tests’ lower sensitivity to Omicron, a singular negative antigen test is insufficient to rule out a COVID-19 infection. Conversely, a singular positive antigen test is sufficient to conclude COVID-19 infection and does not require PCR confirmatory testing. As such, frequent, or serial, testing is recommended for asymptomatic individuals in order to have a significant impact on reducing COVID-19 transmission. The Advisory Table briefing recommendsA testing frequency of three times per week will provide organizations and individuals an effective level of risk mitigation in asymptomatic populations, however, those looking for a higher level of risk mitigation could employ a testing frequency of five times per week.
PCR confirmatory testing
With the current state of affairs, there remains a high pre-test probability of disease, so a singular positive antigen result is sufficient at this time to confirm presence of COVID-19. As case rates continue to decline, the pre-test likelihood also decreases. Once the reported positive rate falls below 50 reported cases per 1 million inhabitants per day, PCR confirmatory testing for antigen positive results should resume. The use of rapid antigen tests in asymptomatic populations should be re-evaluated once there is no evidence of exponential growth and if case rates fall to 20 cases per 1 million inhabitants per day as continuing with such testing is not likely to be useful in reducing transmission.
Rapid antigen tests continue to be a valuable resource to quickly identify and isolate those with COVID-19 infections and further the collective progress towards the end of the pandemic. Considering the recent findings, individuals and businesses are strongly recommended to adopt the updated rapid antigen testing strategies including the combined oral-nasal sampling method and serial testing where resources permit.
Jüni, P., Baert, S., Corbeil, A., Johnstone, J., Patel, S. N., Bobos, P., … McGeer, A. (2022, February 10). Use of rapid antigen tests during the Omicron wave. https://covid19-sciencetable.ca/sciencebrief/use-of-rapid-antigen-tests-during-the-omicron-wave/