A capacity issue in the National CervicalCheck Screening Service has led to the samples of 180 women going out of date, and women will be informed that they need to be retested, RTÉ News has established.
In the 180 cases, HPV was found in the women's samples, but they were not sent on for cytology examination, within the window period of 42 days.
The women are due to be contacted this week, along with their GPs, to arrange retesting.
The HSE has said the expiry happened "because of a delay in a sample processing centre due to Covid-19 restrictions".
CervicalCheck has told patient support groups that there are always some expired samples and that the risk to women is very low.
The problem has arisen under the new HPV (human papillomavirus) screening system, which replaced the old screening service, in March 2020.
The more modern system screens for the HPV virus, which can cause cancer. As a result, fresh samples are needed.
The issue arose at a testing facility in Santry in Dublin.
In correspondence, to be issued to the women and their family doctors this week, CervicalCheck will say that the virus was found in their sample and that another test will be needed, as their original sample was allowed to expire before being sent on for cytology, which is an examination of the cells.
The retest will need to be done at least three months after the initial sample was taken to allow sufficient time for cell regrowth.
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The problem was discovered recently during routine monitoring when the National Cancer Screening Service conducted a site visit to check the Santry centre for quality assurance.
It has arisen out of around 200,000 samples taken during the period in question.
The number of samples taken has increased in recent months, resulting in the capacity problem.
Speaking to RTÉ's Morning Ireland, the clinical director with CervicalCheck Nóirín Russell said the repeat samples for the women affected will be fast-tracked through the laboratory cytology process.
Ms Russell said that 2021 has not been a usual year and that a huge uptake in screening saw 200,000 samples sent for testing in the first half of the year, 50,000 more than was expected.
She said that the vast majority got fully processed and fully screened, but about 200 samples were processed for the HPV virus, but the follow-up cytology testing of cells did not occur as the samples had expired.
A number of patient organisations have been notified to prepare for the news, including the Irish Cancer Society and the 221+ CervicalCheck Support Group.
GPs are being asked to contact the women directly to explain the reason for the repeat test.
Mr Russell said: "We felt because it was more than the average and was increasing [this] was an issue, we felt it was important to prepare the women before the letters went out."
She said there is no quick fix to resolve the capacity issues, and the service is trying to work closely with sample takers to steady the flow of samples so that "peaks and troughs" can be avoided.
CervicalCheck says that the presence of HPV does not mean a woman has cervical cancer and that in most cases it takes 10-15 years to develop this cancer.
In March 2020, the HPV screening system was introduced, as a modern overhaul of the old system and after a series of controversies, inquiries and court cases, involving the national CervicalCheck screeing programme.
Screening is provided for women aged 25 to 65 years, every three to five years. High-risk women are referred for colposcopy.
At this point, 80% are usually found not to require treatment but may need future follow-up. Anyone seeking further information can contact the national Screening Service at 1800 454 555.
Screening Service Chief Executive apologises over expired samples
The chief executive of the National Screening Service has apologised for the failure to test cervical samples that were HPV positive before they expired in the cases of 180 women tested earlier this year.
Speaking on RTÉ's Today with Claire Byrne programme, Fiona Murphy sought to reassure women whose smear tests were positive for the HPV that it is not a confirmation of cervical cancer, but is a risk factor for the disease that requires further testing.
Ms Murphy apologised for the delay in carrying out the second cytology tests in these cases and stressed that the timeline involved in re-testing is low-risk.
"I can only say sorry that we didn't do that for these two hundred women and we have looked at every single step along the way to see how we can speed up the process," she said.
She said patient advocate groups were informed about the issue last week and personalised letters are ready to be sent to the women affected and to their GP's.
"We won't hide anything ... and we have made a mistake and hands up, 'mea culpa'," she said.
Ms Murphy also sought to reassure the many thousands of women who are awaiting results that are not part of these tests, that they will receive their results within the normal eight-to-ten-week timeframe.
Ms Murphy said that the service has looked at how to speed up the process, adding that "part of the trust and confidence is to tell people".
"We have caught up and while we made a mistake it is important to say we have fixed it," she said.