What’s the difference between rapid and PCR COVID-19 tests?
Posted Oct 15, 2021 7:30 am.
Last Updated Oct 15, 2021 7:42 am.
CALGARY – As the fourth wave of the COVID-19 pandemic continues, many Alberta employers are requiring antigen testing–or rapid testing–of their staff prior to working on-site.
But, just how different can these results be compared to the lab-based, gold-standard PCR test?
“When administered correctly, in a person with symptoms, rapid tests can identify 75 to 80 per cent of infections,” explained Alberta Health.
“In a person without symptoms, rapid tests can identify 20 to 60 per cent of infections.”
Whether you have symptoms or not, Alberta Health explains PCR tests can identify 95 to 99 per cent of infections when administered correctly.
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There’s a key difference between the tests, while rapid test results are produced more quickly, they aren’t as sensitive.
“Lower sensitivity means that the tests require more virus for a positive result than the gold standard PCR test,” said the provincial health authority.
“This means the rapid antigen tests might produce a negative result even though there is an active infection. This is a false negative.”
However, Alberta Health states, as with any diagnostics testing, there are several factors that could contribute to false negatives including:
- “Stage of disease when the sample is collected;
- the sample type collected;
- the quality of the sample collected;
- the analytical sensitivity of the test, while extremely sensitive, is never 100 per cent – the rapid tests have lower sensitivity than PCR tests so they will have a higher rate of false negatives than PCR tests;
- the clinical sensitivity of the test.”
The health authority’s advice for anybody who has COVID-19 symptoms is “they should receive a test at an Alberta Health Services’ assessment centre, even if they have done a rapid test and it has been negative.”
