The American laboratory involved in the CervicalCheck smear test scandal used the wrong procedure to re-examine samples, it was revealed yesterday.
New Jersey firm Quest Diagnostics tested the samples from the time they arrived in the laboratory rather than the time they were taken by GPs, the HSE told the Herald.
It means the 6,000 women who are now to be invited for re-testing go back years.
They were screened between mid-2015 and late last year.
Dr Peter McKenna, clinical director of the HSE's Women and Infants Health Programme, said yesterday that the women involved would be contacted at the end of next week.
"Our clinical review has assured us this issue poses little risk," he said.
Since 2015, cervical smear samples that screen positive for low-grade abnormalities on a standard test have been re-examined by Quest Diagnostics to find out if the woman has the HPV virus.
If the test is positive, the woman is advised to go for further investigation.
The HSE said yesterday that Quest Diagnostics was the only one of its three laboratories that was not following the proper protocol.
Quest Diagnostics brought the error to the attention of CervicalCheck in November and it has been doing a lookback since.
Asked why it took three years for this to come to light, and why CervicalCheck had not discovered it through its own surveillance, a spokeswoman said it had "guidelines in place for all laboratories".
She said it also required laboratories to undertake and demonstrate their own quality assurance and that Quest Diagnostics identified the problem through this process.
Dr Gabriel Scally, who carried out a scoping inquiry into CervicalCheck following the controversy that erupted last year, described quality assurance procedures by the screening programme as "non-existent".
The revelations about Quest Diagnostics are separate to the rising concern about the ongoing backlog of tests that are leaving some women waiting six months for results .
CervicalCheck said 84,000 women took up the offer of free tests, which were aimed at reassurance as the controversy raged.
A backlog of about 82,000 are still being processed.
The tests were offered to all women after the Vicky Phelan High Court case in April last year.
This revealed the existence of audits showing some women who went on to develop cancer had received wrong test results.
Fianna Fail leader Micheal Martin, who raised concerns about the backlog in the Dail earlier this week, said yesterday: "I asked a very basic question: Why did [Health] Minister Simon Harris go against official and expert advice at the time when he made the announcement that he was offering free smears to everybody?"
A spokeswoman for Mr Harris said he met with the HSE to discuss the latest issue regarding HPV testing yesterday.
He was assured by the HSE that the offer of a repeat smear test is a "precautionary measure" and that there is an "exceedingly low clinical risk" to the women involved.
"The minister was made aware in December of a potential issue regarding HPV testing and that work was under way by the HSE to assess if there was action required," she said.
"He sought regular updates on all matters relating to CervicalCheck, but to date, a final report on the HPV testing issue has not been received."
The issue has been discussed at the CervicalCheck steering committee, which includes patient representatives, and referenced in weekly reports.
"However, it is important to state the full report from the HSE is not available and was not made available to the committee members," added the spokeswoman.