1 in 5 throat swab tests may be ‘off’

HEALTH chiefs have admitted that as many as one in five throat swab tests for the Covid-19 virus may deliver false negatives.

HSE officials yesterday said that the current sensitivity of Covid19 testing, which involves a swab at the back of the throat or nasal passage, means that patients could be given a false negative in their results, adding that no single test is 100% sensitive.

However, infectious disease consultant at the Mater Hospital Dr Jack Lambert has said that this sensitivity rate is standard for these kinds of diagnostic tests and if someone still has symptoms after a negative test result, they should still self-isolate.

He told the Mail that they’ve been aware of the limitations of the test since February and so ‘we don’t hang our hat on a test’. He said that if someone comes in sick, displaying symptoms which point to Covid-19, with a negative test they still treat that person as if they have the virus.

‘That was the kind of lesson we learned a long time ago. Sometimes the test is not 100% because most of the Covid is down in the lungs, not up in the nose or throat where you are swabbing,’ said Dr Lambert.

Virologist at Trinity College Dublin Dr Kim Roberts said that the 70% sensitivity of the test is what ‘we would normally expect for these diagnostic assays’.

The HSE Covid-19 test uses a technology called PCR which detects pieces of the virus in mucus samples.

The results of the Covid-19 test are not given on the basis of being positive or negative for the virus but rather whether the virus is detected or not. Which means that as the infection develops over the course of a few days the better the chances of detecting it become. ‘It’s not that this is any better or any worse,’ Dr Roberts told the Mail.

The HSE said that this level of sensitivity in Covid-19 testing is ‘not unexpected’ because the virus predominantly affects the lower respiratory tract.