Ireland’s coronavirus testing system delivers a maximum of one false positive for every 500 tests carried out, according to Dr Cillian De Gascun.
The Chair of Ireland’s COVID-19 Expert Advisory Group was speaking after officials announced 248 new confirmed cases and five further deaths.
The latest data form the HPSC shows that Donegal’s 14-day virus rate is now at 178.4, the highest in the country.
Dublin has returned 148.7 confirmed cases per 100,000 people in the past two weeks, while the third-highest figure is in Louth at 94.7.
In a Twitter thread earlier this month, Dr De Gascun noted that standard PCR tests can produce somewhere in the region of 1% to 3% false positive results – leading to claims Ireland could be wrongly diagnosing high numbers of people.
Speaking to On The Record With Gavan Reilly this morning, he said the tweet has, “kind of come back to it to bite me in some respects” and noted that when it comes to Irelland’s COVID-19 testing system, the highest possible percentage of false positives is 0.2%.
“The percentage that I used in that setting really was around, I suppose, PCR in general so across the whole gamut,” he said. “I wasn't speaking specifically about SARS COV 2. I was just trying to give people some background or some context around PCR testing.”
A short thread on PCR #SARSCoV2 There’s been a lot of discussion recently about the use of PCR (Polymerase Chain Reaction) for the diagnosis of #SARSCoV2 infection, and the interpretation of its results. 1/n
— Cillian De Gascun (@CillianDeGascun) September 13, 2020
Dr De Gascun said the manufacturers of the serological assays – or tests – used in Ireland report a specificity of “in or around 100%.”
“Now we do know that, generally speaking, in the real world, assays are probably never quite 100% specific,” he said.
“If we take some real-world data of our own, we can look at the serial testing programs that we've carried out.
“So, the serial testing programs have generated a positivity rate in the region of 0.2%. Those people will be asymptomatic, they would have no symptoms and that's why it gives us a really good indication about the performance of the assay.
“So, by definition, our false positive rate can't be more than 0.2% because we have that real-world data from the serial testing programs.”
He said the data from the serial testing programmes shows “that the false positive rate is certainly not something that's driving the pandemic in Ireland.”
“It's really important to try to highlight that, you know, we have people going into hospital now, the number of people in intensive care is increasing, the number of deaths is increasing and PCR isn't driving this,” he said.
“I think I was keen to highlight in that thread, sort of, the limitations of it but at the same time it's a really good test that we have at this point in time, and it's certainly not driving at case numbers here.”
Dr De Gascun also said that Ireland’s testing capacity may be increased over the winter if cases continue to rise.
Ireland officially has a testing capacity of 100,000 per week; however, that number has never actually been hit – with 88,079 carried out in the last week.
“As we are about to go into a winter, we may need additional capacity and that is something the HSE is looking at,” he said.
“The 100,000 capacity was never really considered necessarily a definitive ceiling. It was always an initial target we wanted to get to on the basis of the numbers back in April and May.
“The HSE continues to keep that under review and, if it needs to increase, it can.”
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