The question: I’ve been reading in the news about all the different tests for COVID-19. Some are done in labs, some are done in schools and some are done at home. Which one is best?
The answer: The ideal medical test would be one that is easy to perform and provides highly accurate results very quickly.
But unfortunately, when it comes to COVID-19, the perfect test does not exist. So we must rely on a combination of approaches to detect the illness and help control its spread.
The two main assessment tools are the PCR test and the rapid antigen test.
PCR stands for polymerase chain reaction. It’s essentially a lab-based method that amplifies small amounts of genetic material to confirm the presence of SARS-CoV-2, the novel coronavirus that causes COVID-19.
PCR technology produces very reliable results when analyzing respiratory secretions from an infected person – often collected with a deep nasal swab about a week after exposure. PCR is accurate about 98 per cent of the time. As a result, it’s considered the “gold standard” of COVID-19 tests.
But, on the downside, it usually takes about 24 hours before you get the results, says Allison McGeer, an infectious diseases physician at Sinai Health in Toronto. “The test has to be performed by trained technicians in a microbiology lab using very expensive equipment.”
By contrast, the rapid antigen test can provide an answer in 15 to 30 minutes. This fast test detects bits of proteins – or antigens – found on the surface of the virus. The testing devices, which vary slightly from brand to brand, are relatively easy to use.
You rub the back of your throat and a nostril with what looks like an extra-long flexible Q-Tip. The tip of the swab is dipped into a special solution, mixed around, and then two drops of that solution are placed on a testing strip. If two lines appear on the testing strip within half an hour, it means the result is positive.
However, while you gain speed you also lose some degree of accuracy. A fairly high concentration of the virus is needed to produce a positive finding with the antigen test. And mistakes are more likely to occur when the test is done by members of the public who are unfamiliar with the procedure.
Your chance of getting a correct result with a rapid antigen test ranges from 50 to 95 per cent. The accuracy depends on a lot of different factors, but a key one is whether you’re symptomatic or asymptomatic.
The test is usually more accurate “in people who have symptoms and who are shedding a lot virus,” explains Dr. McGeer.
But this huge variability means you could still be infected even if your test is negative. In other words, you can’t always rely on the findings.
So, if you have COVID-19 symptoms, or you have been exposed to an infected person, you really need a reliable PCR test to confirm a diagnosis.
Last year, you may recall that long lines snaked outside of COVID-19 assessment centres where medical staff performed deep nasopharyngeal swabs – “brain ticklers” – and the samples were then sent off to labs. As the demand for PCR tests grew, it often took several days to receive the results.
The time-consuming visit to a testing centre was a huge inconvenience for many people – especially families with young children, says Irfan Dhalla, co-chair of the federal COVID-19 Testing and Screening Expert Advisory Panel.
“Taking a child who has a fever or sore throat all the way to a testing centre could be quite difficult,” he adds, noting that “some parents had to book time off work.”
Since then, lab capacity has been ramped up so it seldom takes longer than 24 hours to process a test.
But possibly more important, various plans are now underway to simplify testing procedures.
In Toronto, for instance, some hospitals have been distributing PRC sample-collection kits directly to schools and daycare centres.
“If a child develops symptoms, the parents can pop over to the school to pick up a kit and the test can be performed at home,” says Janine McCready, an infectious diseases physician at Michael Garron Hospital.
Once the test is completed, the tip of the swab is sealed in a plastic tube and dropped off at the school or a hospital location where it’s then sent to a lab for PCR analysis.
What’s more, the test has been modified to make it easier to do. It involves just a swab of the mouth plus inside the nostrils. “It’s not uncomfortable,” says Dr. McCready. “Most kids can do it themselves.”
An equally easy-to-use PCR saliva collection kit is being provided by the Hospital for Sick Children. “They literally spit into a tube,” says Julia Orkin, physician lead of the school-based testing program at Sick Kids.
The goal of the kits is to lower the barriers to testing, And hopefully, they will lead to the identification of more COVID-19 cases so steps can be taken to prevent the virus from spreading to others.
Indeed, as the new school year begins, there are growing concerns that increased transmission of the highly-contagious COVID-19 Delta variant may lead to frequent classroom closures.
“More accessible testing, and a smarter testing strategy, should be able to keep more kids in school for more of the time,” says Dr. Dhalla.
He thinks that the rapid antigen test has a role to play as a screening tool in schools. Although it’s not as accurate as PCR, it can help identify outbreaks if done routinely, possibly several times a week. Its key advantage is that it produces results in just 15 to 30 minutes. So, if a potential case turns up, corrective action can be quickly introduced, such as keeping an infected child at home.
Rapid antigen tests have been used in certain workplaces. Companies can obtain supplies of the devices through various government-funded programs. In Ontario, private schools are eligible to receive the tests free of charge and some of them are already planning to set up their own rapid-screening operations. “The staff at the school will need to manage the distribution of the tests and handle the results,” says Dr. Dhalla.
Meanwhile, Ontario will be requiring unvaccinated school and childcare staff to undergo rapid antigen tests twice a week. The antigen testing devices will be provided through school boards and local pharmacies.
Dr. McGeer says effective testing and screening practices should prevent some school outbreaks. But, she adds, such efforts need to be matched with more vaccinations.
She points out that children below the age of 12 are not yet eligible for a shot. If we want to protect the very young, we must curb the spread of the virus in the community – and vaccines are a crucial part of that strategy.
Paul Taylor is a former Patient Navigation Adviser at Sunnybrook Health Sciences Centre and former health editor of The Globe and Mail.