"Mistakes happen and this is one of those very unfortunate, devastating times," said Chris Power, president and CEO of the Capital District Health Authority, which is responsible for health care in the Halifax area and part of a neighbouring county.
In late April, pathology results were switched on charts for two patients, leading to one patient needlessly losing a breast and the other not being scheduled for the necessary surgery.
In the second case, which happened a month later, Power said a diagnostic biopsy was conducted on the wrong patient after slides were mislabelled with incorrect patient identification.
Asked why the health authority waited nearly four months to go public with the information, Power said: "We were sensitive to (the patients') needs and we wanted to be sure that they got through this part of their treatment and care before we made it public."
The errors affected four patients, two who were misdiagnosed with cancer and given needless treatment, and two who were mistakenly told they did not have cancer and failed to receive the treatment they required.
The mistakes were detected in laboratories at the Queen Elizabeth II Health Sciences Centre in Halifax, where samples from the entire health authority are collected to be processed and analyzed.
An oversight system — referred to as a quality assurance mechanism — compares all processed tissue from before and after a surgery to ensure they are the same.
"What's supposed to happen is they match," said Power. "But in this case they didn't and that's when we were alerted that we have a problem here."
The health authority said in a news release it is confident it has identified the only affected patients after reviewing other tissues taken during that time period.
Since the incidents, Power said Capital Health has conducted two internal reviews, reviewed standard operating procedures and investigated best practices worldwide.
"As devastating as this was for our patients it's been a hugely difficult time for our staff who were involved as well," said Power. "Nobody wants this to happen."
Power said the introduction of bar codes later this year and an automated laboratory system in 2014 will lower the likelihood of similar mistakes happening in the future.