Rapid screening tests: a more detailed guide would minimize errors

Jean-Benoit Legault, The Canadian Press

MONTREAL — Users of rapid antigen-detection tests for COVID-19 are making several mistakes when handling and interpreting the results, Montreal researchers have found, but the situation can be remedied by giving them more detailed instructions.

Some of the most common mistakes were the amount of fluid to use and the identification of a positive result, especially when the line attesting to the presence of the virus is particularly weak, said doctor Cédric Yansouni of the McGill University Health Center research institute. .

Doctor Yansouni recalls that during the pandemic, several countries rushed to deploy these self-tests without first verifying that users could use them and interpret them correctly.

“It was necessary to determine how much we can rely on the performance of people doing their own test, and that’s what we tried to do,” explains Dr. Yansouni out.

A team visited companies where at least two active cases of COVID-19 were identified by Montreal Public Health. The team visited each company twice a week for two weeks.

At each visit, the study staff instructed the participants in the nasal swab taking procedures and provided them with either the manufacturer’s manual or a modified quick reference guide, without providing further details.

A total of 1,892 tests were conducted on 647 participants.

At the first self-diagnosis visit, 55.6% of participants who correctly received the modified manual identified a weak positive test result, compared to 12.3% of participants who used the manufacturer’s manual. Similarly, for a positive result, the correct identification rate increased from 51.5% when using the manufacturer’s guide to 89.6% when using the modified guide.

Improvements were also seen with a strong positive result and with an invalid result.

“In the amended instructions, there was a lot of emphasis on interpretation, so not just one example of a positive and one example of a negative,” Yansouni said.

“We invited people to systematically ask themselves, ‘Is the test valid? Is the test negative? Is the test positive? Here are examples of barely visible lines that are nevertheless absolutely positive’. And when we used that with several untrained people, we saw a dramatic difference.”

With an extremely simple intervention, he adds, “we have been able to improve the interpretation of the majority of the positive results. With a simple intervention, untrained people achieve a confidence level that is (approaching) that of a professional”.

The researchers did not try to find out what caused the users of the self-tests to misinterpret the results, for example in the presence of a faint line indicating a positive result. However, Doctor Yansouni reminds us that the human eye is not infallible. There may also be a psychological component to the phenomenon.

“The first step, he said, is to know the magnitude of the reading bias being introduced with at-home testing because I don’t believe rapid testing for COVID will be the last testing we use. The biggest challenge for the authorities is not only to deploy them, but also to find the right way to communicate with the public about the correct interpretation of the test, which will change according to the epidemiological situation. And that is not easy.”

At the time of the study, the rapid tests had not yet been approved by Health Canada for self-use by untrained individuals. The vaccine was just starting to roll out and there were multiple outbreaks in the workplace.

The results of this study are published by the medical journal JAMA Network Open.

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