Eastern Health said it discovered a problem on Dec. 9, when it found a test result from its lab did not match with one during a quality assurance test. The health board said it stopped its testing at the lab and sent new tests to an external lab for follow-up.
Eastern Health CEO Vickie Kaminski apologized for the inaccurate tests, which were intended to determine the best course of treatment for the patients.
"If the results are incorrect, it is very distressing to the person involved and to us as a health authority," Kaminski said in an interview from St. John's.
"We strive to provide excellent patient care and in this case we didn't do our best."
Health Minister Susan Sullivan also issued an apology.
"As health minister, I never want to hear that patients in our health care system have been negatively impacted by receiving a treatment they didn't require," she wrote in an email.
Kaminski said patients received Herceptin, a drug that in a small percentage of cases can cause side effects for the heart, liver and lungs.
"You should never receive that kind of a strong agent if you don't need it," Kaminski said.
She said oncologists have told her the patients who were given Herceptin haven't suffered side effects but doctors will continue to monitor them.
The health board has launched an investigation to determine what caused the false results. Kaminski said the findings from that investigation will be made public and similar breast cancer tests will be sent to an outside lab in the meantime.
After the initial error was detected in December, 65 test results were examined and of those, 34 were sent to a lab in Miami on Jan. 31 to be reviewed, Eastern Health said.
Of those, nine were confirmed to be incorrect on Tuesday and the patients involved were informed Thursday, Kaminski said.
Eastern Health has been under scrutiny in the past for incorrect breast cancer test results.
Between 1997 and 2005, hundreds of Newfoundland women and men received inaccurate breast cancer test results, which led to a public inquiry that identified major problems throughout the province's pathology system.
Kaminski said the public inquiry that resulted from those errors taught the health board lessons in quality assurance that helped it catch the latest problem.
"The quality assurances we put in place have prevented us from going down a road of hundreds of wrong tests," she said.
Kaminski said it took eight weeks to inform patients after the first problem was detected because the quality assurance program requires double checks after an initial error is found.
She said it also required some time to find an expert to verify the tests in question and allow the lab to complete its analysis.
— By Michael Tutton in Halifax.