An inquiry into the screening of tissue samples for cancer at a hospital has found errors were made in 2,459 cases.
In 181 instances, the errors at Wrexham Maelor Hospital could have had a "definite influence" on treatment said the Health Inspectorate Wales (HIW).
North Wales NHS Trust said changes at the unit had been made.
The pathologist at the centre of the investigation retired before the review and the HIW said he declined to take part in their investigation.
Just three errors were first identified at the unit by another consultant in January 2008.
But it led to a massive media campaign to indentify those who had their samples screened by Dr Roger Williams between 2004 and 2007.
CASE STUDYYour world suddenly seemed to have turned upside-down.
It did bring a lot of fear back, a great deal of fear, not only for myself. It was awful for the family, because obviously I was distraught.
I was working, so trying to sort of cope with the pressure of work, going out and doing my job without feeling angry as well - angry and tearful at the time, very angry.
I think in the position that this person was employed within, there should be absolute trust in their ability to diagnose correctly.
Mrs Charles had been given an all clear after biopsies for stomach, bowel and breast cancer and had to have her case reviewed.
In all, 11,671 "all-clear" samples sent to the hospital had to be re-examined.
Of those reviewed, 660 were noted as showing discrepancies with the original report that might have had an effect upon clinical management, and 472 patients were identified as requiring further clinical review.
Ninety patients have since had a change in their prognosis and/or medical treatment or management, North Wales NHS Trust said.
After the case emerged, Dr Williams, who was locum at the histopathology unit at the time of the errors, told a newspaper that he was "deeply sorry for what has happened and for any harm that may have been caused to patients".
However, the former medical director of the then North East Wales NHS Trust, refused to take part in the investigation.
"The particular consultant histopathologist concerned in the misreporting of histopathology results had retired prior to our review," states the HIW report published on Friday.
"Although we invited him to contribute to the review he declined to do so.
"As a result we have been unable to consider any direct evidence from that consultant in respect of factors relating to his working environment or practice which might have had a bearing upon our findings."
It is known that at least two of those initially given the all-clear from cancer have since died.
CHANGES MADEHowever, hospital officials insisted in 2008 that the two patients had been successfully diagnosed by other means, and the treatment they received would not have been any different.
The inquiry findings said that the errors uncovered at Wrexham Maelor were broken down in to three types, with 181 samples identified in the most serious category.
A further 479 were labelled as less serious category 2 mistakes, while 1,799 samples were identified in category 3.
The 33-page HIW report highlights a series of concerns at the screening unit, including a lack of team work among consultants, increased workloads since 2004, and a laboratory building that has "long since passed optimal usefulness".
It also raises the issue of "double reporting", the process where another consultant double checks tissue sample results.
The HIW said at Wrexham Maelor this only happened when a sample was initially thought to be cancerous.
LEVEL OF ERROR
The inquiry suggests that the North Wales NHS Trust should now consider whether biopsies giving the all-clear should be seen by another pair of eyes.
In all, the investigation makes 12 recommendations to address the failings it found at the unit.
But the report authors admit that they still do not know for sure why the mistakes happened.
"In the absence of evidence from the particular consultant concerned in the erroneous reporting of results, we can draw no definitive conclusions as to the reasons for the errors occurring," states the inquiry.
The North Wales NHS Trust apologised for the errors and said changes recommended by the report have been made.
"We offer our sincere apologies to patients and their families for any distress that this matter will have caused. The care and well-being of our patients remains our highest priority," it said in a statement.
"We acted as quickly as possible upon discovering the issue and implemented a full and thorough review of the Histopathology Department, its staff and procedures."
Changes brought in include a more robust system for double-checking cases, especially for complex or negative cases.
New equipment has also been brought in for the laboratory, including multi-headed microscopes, allowing simultaneous review of samples by two or more consultant histopathologists.
The trust added: "We would like to reassure patients that the changes that have been implemented within the department will minimise the risks of such errors happening again."
Solicitors involved in seven cases of patients who were wrongly given the all clear said patients and their families needed reassurance.
Two of the patients have since died.
Solicitor Kevin Saul said: "Our clients deserve a full explanation, safeguards in the system and an assurance that this sort of terrible error can never happen again."
RELATED INTERNET LINKS
Health Inspectorate Wales
North Wales NHS Trust
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