Some researchers and governments are questioning the accuracy of rapid antigen tests to identify Omicron – and it has been suggested that rapid tests based on throat- instead of nasal swabs might be more accurate in detecting the highly infectious COVID-19 variant.

Last week, a small pre-print study reported that 29 fully vaccinated and boosted individuals who became infected with Omicron during December were diagnosed by PCR tests three days sooner on average than testing positive with antigen tests (also called lateral flow tests). 

The study, which tracked COVID-19 in five workplaces in New York and California through daily employee testing, used saliva-based PCR tests and antigen tests using nasal swabs.

Lead author Dr Blythe Adamson told Health Policy Watch that the median time from the first positive PCR to the first detectable antigen-positive test was three days and that the viral load appeared to peak in an individual’s saliva as much as one to two days before in their nasal cavities. 

“We did epidemiological investigations and contact tracing and had four confirmed transmissions in the time period between phase zero and one, when the antigen test was negative and the PCR was positive,” added Adamson.

All individuals developed symptoms within two days of the first positive PCR test results, but the antigen tests only revealed a positive result after they developed symptoms. 

UK sticks to rapid tests

The US Food and Drug Administration (FDA) recently warned: “Early data suggests that antigen tests do detect the Omicron variant but may have reduced sensitivity.” 

However, the FDA acknowledged that antigen tests “are generally less sensitive and less likely to pick up very early infections compared to molecular tests”, adding that “if a person tests negative with an antigen test but is suspected of having COVID-19, such as experiencing symptoms or have a high likelihood of infection due to exposure, follow-up molecular [PCR] testing is important for determining a COVID-19 infection”. 

Meanwhile, the UK Health Security Agency reported last month that the antigen tests it was using “indicate a comparable sensitivity [to Omicron]  to that observed for previous strains of SARS-CoV-2 including Delta, which has been the predominant strain in the UK from May to December 2021”.

Rapid tests are accurate once symptoms start

A Cochrane review of 64 studies measuring the accuracy of antigen tests published in March last year, before Omicron emerged, found that the rapid tests detected almost three-quarters of COVID-19 cases once people showed symptoms but only 58% of cases in people without symptoms.

“Tests were most accurate when used in the first week after symptoms first developed (an average of 78% of confirmed cases had positive antigen tests). This is likely to be because people have the most virus in their system in the first days after they are infected,” according to the review.

“In people who did not have COVID-19, antigen tests correctly ruled out infection in 99.5% of people with symptoms and 98.9% of people without symptoms.”

Meanwhile, in the face of its growing COVID-19 caseload, Israel recently switched to home testing, opting to reserve PCR tests for older people. However, this week it urged those who have come into contact with someone with the virus to take an antigen test 72 hours after exposure rather than 12 to 24 hours and asked people to put the swab in the throat and not just the nose.

Scientists believe that one of the reasons the antigen tests may be less effective is that Omicron infects the throat more than the lungs so throat swabs or saliva tests would be more effective.

“The method of testing only from the nose probably misses Omicron,” said Dr. Gili Regev-Yochay, director of the Infectious Disease Epidemiology Unit at Sheba Medical Center in an interview with Israeli media. “The swab should be inserted into the pharynx and from there into the nose, and only then will the sensitivity increase.”

However, most health professionals have been slow to give such advice for fear that users would swab their throats too deeply or in a way that could cause them harm. 

“You have to use the right test at the right time,” Adamson continued. “Omicron will not be the last variant. We need to make sure we are ready and able to better identify variants as they come.”

Image Credits: Alia Ameen/Twitter .