Test results for about 40,000 patients in Ontario have been thrown into doubt because of mistakes made by a hospital pathologist.

Grey Bruce Health Services in Owen Sound said Wednesday it had found an error rate six times higher than normal in recent diagnoses by Dr. Barry Sawka, whose main task was diagnosing diseases from patient samples, mainly looking at biopsies to determine cancer.

Hospital officials speaking to the media on a conference call Thursday morning said the errors Sawka made include missed cancer diagnoses.

Sawka started working for the local health services in 1994. The flaws in his work were first noticed in February. As soon as the errors were discovered, hospital officials asked Sawka to stop practising, but no one immediately informed the public about the errors.

Officials said they wanted to wait until a hospital in London, Ont., finished an independent review of Sawka's most recent diagnoses. The results of that review came on April 30 and confirmed that Sawka made clinically significant errors in 35 cases in the months before he stopped practising.

Grey Bruce Health Services CEO Pat Campbell said doctors have already started to change treatment for some patients who were misdiagnosed.

All of the cases that Sawka handled in his 14 years — about 40,000 — will be reviewed. Campbell apologized for any problems the errors may have caused to patients, and said she's aware the news is going to cause anxiety to people whose tests were handled by the hospital.