Fast and (not) easy: the challenges of rapid testing

COVID-19 rapid tests are widely viewed as a key tool in managing outbreaks, particularly in children as winter approaches, and the highly infectious Delta variant remains the dominant strain in Canada.

Yet how to best use rapid tests, which can deliver results in 15 minutes, continues to be debated, even among experts, due to their limitations.

Canada has a provincial patchwork of approaches to school settings; a lot put together in recent weeks. This is despite rapid tests now widely used in the workplace, in part supported by the federal government, which has purchased and distributed millions of tests.

“Until there, [rapid tests] have been used in adult settings to keep businesses open with enough success, ”says Jim Kellner, a Calgary pediatrician specializing in infectious diseases at the Alberta Children’s Hospital and a member of the Federal COVID-Immunity Task Force. 19, which has helped inform government policy on infection and immunity issues.

Dr Kellner notes that there is little debate around the usefulness of these tests – more than 20 approved to date by Health Canada – including Abbott’s Panbio, which is used for screening workers and workers. unvaccinated travelers. However, their effectiveness in screening large unvaccinated groups, such as children aged 5 to 11, is less clear.

“The debate over these tests has been going on not only in Canada but around the world for over a year now, and it still has the same dimensions,” adds David Naylor, co-chair of the COVID-19 Immunity Task Force, contrasting Canada with the United States, which allowed liberal use of rapid tests with mixed results.

Understanding the trials

The advantages of rapid tests, often referred to as antigen tests, are that they can be administered by a parent to a child, for example, involving a gentle nasal swab, while still providing quick results on the spot.

In contrast, widely used PCR (polymerase chain reaction) tests typically involve an uncomfortable deep nasal swab, often administered by a nurse, with samples sent to a lab and results taking a few hours, at a minimum.

“This speed of execution and relative ease” makes rapid testing attractive during “sniffle season,” says Dr. Naylor, especially for parents who are wondering if their child has a cold or COVID-19. He says some parents have banded together to buy large batches of tests in the absence of government policy.

Although they are generally accurate when symptoms are present, rapid tests are less effective at identifying viruses in asymptomatic populations with low incidence of the disease, adds Dr. Naylor.

This is due to their tendency to produce false positives (results indicating disease when there is none) and, to a lesser extent, false negatives (results showing no virus present when it is. actually), compared to PCR testing.

PCR tests, which can detect the genetic material of COVID-19, largely produce fewer false positives and false negatives and, therefore, are often needed to confirm positive results from rapid tests.

Rapid test technology differs, detecting unique proteins on the surface of the virus – called antigens – and may not detect all asymptomatic cases when little virus is present. Additionally, rapid tests are known to not have as high a specificity – a scientific term for the ability to identify disease-free samples – as PCR tests. As a result, rapid tests produce more false positives.

“The performance of (rapid tests) in the real world depends on the prevalence in the population,” says Barry Pakes, program director of public health and preventive medicine at the Dalla Lana School of Public Health at the University of Toronto.

For example, for every 10,000 rapid tests in a population with a virus prevalence of 0.1% (or 10 actual cases), there will likely be a few false negatives and hundreds of false positives, says Dr. Pakes. “So how it is deployed as a program is critically important. “

That’s why Dr Pakes says rapid tests are more effective at helping control outbreaks in schools or tracking the spread of the virus in areas with high prevalence and low vaccination. A higher incidence results in more true positives than false positives, he explains.

How the provinces differ on the debate

Despite an agreement on this front, the provinces have not taken similar paths using rapid tests. In Quebec, for example, they are offered in schools, administered by staff for symptomatic unvaccinated children. In Alberta, rapid tests have been made available to parents to test symptomatic children at home in outbreak areas, while in Ontario, the tests are now used in school outbreaks or in high-risk communities. incidence in schools for asymptomatic unvaccinated children.

In contrast, as of mid-November, British Columbia had yet to offer rapid school-based screening for children, while Manitoba had mandated screening only for unvaccinated staff. Meanwhile, Saskatchewan and Nova Scotia have provided rapid parenting tests for children under 12.

“Despite efforts to provide a broad framework at the federal level, we never seemed to achieve national consistency on how this should be done,” said Dr Naylor.

The situation in schools

He notes that he is “more of a hawk” for using rapid tests more widely for unvaccinated children than some colleagues, “but I fully respect those who are more careful.”

Calgary’s Dr Kellner points to new evidence suggesting that rapid tests, despite their limitations, may be effective in controlling outbreaks. He mentions a British study, published in The Lancet in September, in which some schools used rapid tests for seven days on close contacts during outbreaks while others quarantined close contacts at home.

“Rapid tests have worked as well as quarantine to control epidemics and have kept many more children in school,” he says of research involving students between the ages of 11 and 18.

This is important because false negatives, while less common, can lead to greater spread. Even here, although antigen tests may miss more asymptomatic cases than PCR tests, research indicates that these people generally have “non-infectious levels of the virus,” says Dr. Naylor.

Routine diets, like those used in the workplace requiring two to three tests per week, would likely detect most infectious cases, notes Dr Kellner.

With vaccines approaching for 5 to 11 year olds, the use case for rapid tests will likely decline, with PCR remaining the key diagnosis for symptomatic cases, he says. However, children aged 4 and under are not vaccinated, so rapid testing will remain important.

“At the very least, it should be offered so that parents can test symptomatic children at home and schools can test symptomatic children if they develop symptoms at school,” he says.

Debate aside, Dr Kellner and other experts agree that rapid testing technologies – already long used for pregnancy, for example – will only become more widely available as accuracy improves and awareness increases. and consumer demand will increase, while the next pandemic is not an issue. of if, but when.

“This is the Fitbit generation, so it stands to reason that these types of tests will become more common.”

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