One Manitoba woman may have had a mastectomy unnecessarily, and several other patients received incomplete diagnoses after their biopsies went missing, a newspaper reported Monday.

The incidents occurred months before officials of Diagnostic Services Manitoba, a nonprofit corporation, said diagnostic errors were "extremely unlikely."

Documents obtained by the Winnipeg Free Press through a Freedom of Information request reveal that health officials launched seven critical-incident investigations into diagnostic errors between July 2007 and March 2008, including a probe into errors made by a veteran St. Boniface Hospital pathologist.

In two separate incidents in July and November 2007, groups of samples and specimens were lost, which prompted exhaustive searches of a pathology lab and various other health facilities.

As a result, several patients did not receive a complete diagnosis and others had to be retested.

Several other patients had to have a second biopsy after a piece of lab equipment malfunctioned in September 2007, the paper reported. Some patients did not receive a complete evaluation as a result.