A doctor who advises Nova Scotia’s health minister said this week's disturbing disclosure that a woman had an unnecessary mastectomy in Halifax shows why Nova Scotians need a way to measure medical errors.

Nova Scotia's 10 health authorities keep records of what they call adverse events.

The largest, the Capital District Health Authority, said during the most recent fiscal year there were over 19,000 reports, but it says only three per cent resulted in harm to a patient.

That works out to almost 600 people. It did not disclose what level of harm was involved.

Capital Health confessed on Monday it made mistakes in two separate instances, both involving cancer patients.

A woman in her 60s had a breast removed when the process was unnecessary and the other patient, who needed surgery, was not scheduled for the procedure until after the mistake was caught.

In a second separate case, tissue samples were switched before the pathology analysis. One patient had an unnecessary diagnostic biopsy and the other patient never got the followup they needed.

The Health Department said since there is no central collection of the health authorities' records they have no overview of the seriousness of the problem,

But if Dr. Peter Vaughn had his way, that would change.

The chair of the Patient Safety Advisory Committee recommends a centralized system for all reports.

"We need to have the information to be able to track and monitor so we can prevent serious occurrences happening," he said.

Vaughn stresses most  incidents don't involve patient harm but as of now there is no way for Nova Scotians to determine how many mistakes are made in their health district or what they are.

He said he’d like to see that information made public if a central reporting system is in place.

N.S. blocks CBC's hospital data request

Last winter Nova Scotia’s health department blocked CBC from gaining access to hospital data in the province.

On Dec. 19, 2012, CBC asked all provinces and territories to allow the Canadian Institute of Health Information (CIHI) — an organization that collects hospital and health-care data — to release information submitted by the hospitals. Provincial and territorial permission is required as part of CIHI's data release policy.

CBC's flagship investigative show the fifth estate planned to use the data for Rate My Hospital, a special investigation that presents Canada's first-ever hospital report card and profiles more than 600 hospitals across the country on its website.

The show wanted to compare eight indicators tracked by Canadian hospitals, including mortality rates on weekends and foreign objects left in patients during surgery.

Deputy health minister, Kevin McNamara, initially approved the release of hospital data with a letter to CIHI dated Jan. 18, but six days later revoked the approval.