Antigen tests detect Covid in just half of asymptomatic cases found via PCR

Antigen Covid-19 tests – the rapid virus test kits sold shops – pick up just half of asymptomatic coronavirus cases that tested positive on PCR tests, a HSE study has found.

The report, produced by the HSE’s Covid-19 antigen testing working group, found that antigen tests were not as reliable as PCR tests – the standard test used by the HSE to detect the virus in people – to rule out infection.

The group warned that for the screening of people who are not showing Covid-19 symptoms, single antigen tests are not recommended, as “a significant proportion of people who are infected and infectious to others will not be detected”.

The use of antigen tests has caused deep divisions, with the National Public Health Emergency Team warning the public that they “cannot rely” on them, while businesses and industry groups back their use to facilitate a wider reopening of society.

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Minister for Higher Education Simon Harris confirmed on Monday that large-scale antigen testing has started in four Irish universities and that this will be significantly increased from September, with mass screening of thousands of students and staff on a regular basis.

Widely used

Antigen testing is widely used in other European countries.

The HSE study found that the rapid test for nasal swabs picked up 52 per cent of cases that were found to be positive by PCR tests, but caught 80 per cent of cases found strongly positive by PCR tests.

“In summary, the antigen test assessed detects about half of the asymptomatic people who are PCR positive when the samples were self-collected under supervision and the testing was performed by trained scientists,” the report says.

“The test detected about four out of five asymptomatic people who are strongly positive on PCR and likely to be infectious.”

The study concluded that even where people have a high viral load, antigen tests could still miss two in every 10 infectious cases where people have no symptoms.