COVID-19 | Good use of rapid tests

Over the past year, a new practice has appeared in some companies: screening employees with rapid tests for COVID-19. Given the risk of false negatives, was it worth it? A Montreal study seems to show that yes, but that we can do better when the test results are ambiguous.

Mathieu Perreault
The Press

Instructions

The McGill University study involved more detailed instructions than the manufacturer’s for the use and interpretation of the tests. “We really see a big difference for the interpretation of weak positive tests, when the second line of the test is pale”, explains Cédric Yansouni, lead author of the study published in JAMA Network Open this week, who is a specialist in infectious disease diagnostics at the McGill University Health Center (MUHC). With the manufacturer’s instructions, employees of participating companies used the test correctly in more than 90% of cases where the test was negative, in almost 85% of cases with a clearly positive test and in almost 80% of cases with a invalid test. With more detailed instructions, the correct use rate exceeded 95% for clearly positive tests. More importantly, proper use with a weak positive test increased from 12% to 56% with these detailed instructions. “Even when the line is weak, a positive test is a positive test,” says the D^r Yansouni.

Test repeatedly

The study had no data on the proportion of false negatives, only on misuse of the test (the false positive rate was less than 1%). But in general, several studies have shown that the rate of false negatives decreases if we do repeated rapid tests. “It is very likely that, among these asymptomatic participants, there were false negatives, says the D^r Yansouni. Think of rapid testing as a test to detect people who are most at risk of being infectious to others at the time of testing. It is still possible that you are infected despite a negative result, and that you subsequently become contagious in the hours or days following a negative result. The best insurance is to do repeated tests to detect as early as possible a person who could transmit COVID to others. » An American study published last September in the Journal of Infectious Diseases showed that if rapid tests are done every three days or less, the rate of false negatives is minimal.

A gray area

Starting in the spring of 2021, companies that cannot work from home, such as factories, used rapid tests to detect their infected employees and send them home to avoid outbreaks. But the use by a quidam of a rapid test was only authorized last December in Canada. “Initially, the rapid tests had to be done in the company by a caregiver, says the D^r Yansouni. Then, we accepted that it be done by an employee trained to do these rapid tests. But it does appear that several companies have given quick tests to their employees to do themselves. Canada was the only country along with Australia to wait so long to allow self-tests, according to the D^r Yansouni. “We have an evaluation system based on clinical use, when it is absolutely necessary to know if a person is infected in order to decide on the right treatment. Another assessment system is needed that takes into account other decisions, such as arrangements to limit outbreaks, for example, or limit risks in social situations. »

13 companies

The study was carried out between July and October last with 13 companies, chosen from a pool of 168 companies having declared outbreaks. Four MUHC teams were dispatched to the companies to supervise the self-tests. In all, 1892 tests were done on 647 participants. The study also evaluated the difference between the performance of the first and the second self-test, and then saw a less marked difference between the detailed instructions and those of the manufacturer. “It shows that people get better if they have to do several self-tests,” says the D^r Yansouni.

Close or test

The current situation makes screening at work much less widespread. But even at the worst of the Delta and Omicron waves, the companies that used them did so only during outbreaks, according to Geoffroy Denis, occupational health specialist at the Montreal-Centre public health department, who collaborated on the study. “I would count on the fingers of one hand the companies that tested their employees all the time, or tested their asymptomatic employees, says the D^r Dennis. With Omicron, too, things have changed because a rapid test can be negative a few days after the onset of symptoms and then positive. Currently, the DSP’s recommendation is that symptomatic employees take two negative rapid tests separated by at least 24 hours before returning to work. »

For her part, Victoria Drolet, head of communications at the Conseil du patronat du Québec, believes that temporary closures are now more frequent. “I didn’t hear that employers were systematically screening their staff,” says Ms.^me Funny. I think it’s pretty rare. Some employers are more cautious and close their establishment for a few days if employees are infected, for example restaurant owners. »