Three years after a woman's breast was removed unnecessarily because she was misdiagnosed with cancer from another woman's test sample, the exact reason for the mix-up remains unknown to investigators.

In two reports on the case issued this afternoon by Health and Disability Commissioner Anthony Hill, he criticises Southern Community Laboratories (SCL) for "unsafe policies and practices" and the Southern District Health Board for delays in telling one of the women involved.

"Patient Y" was 57 in 1999 when first diagnosed with breast cancer - in her right breast. She had surgery in 2000 to remove the cancerous and surrounding tissue and lymph nodes in her armpit, followed by radiotherapy.

On January 25, 2012, following mammogram detection of abnormalities in her left breast, a biopsy (tissue sample) was taken. Cancer was diagnosed by SCL and she had a left mastectomy in March 2012.

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Analysis by an SCL pathologist of tissue taken during the surgery did not show evidence of cancer. This prompted her to investigate further.

Also on January 25, 2012, a second woman, "Mrs X", then aged 54, had a breast tissue sample taken and analysed at SCL. The reported finding was that she had a benign lump in her right breast. However, doctors at the DHB thought this inconsistent with other clinical information and asked for a second biopsy.

Two samples were taken in February 2012 and this time cancer was diagnosed in Mrs X. The following month she had both breasts removed.

An SCL pathologist, in reviewing patient Y's samples, noted the initial, benign specimens from Mrs X and her later samples which, the pathologist told the commissioner's office, "was diagnosed as invasive lobular carcinoma.

"On review," the pathologist continued, "this tumour looked remarkably similar to that seen in the initial tissue submitted on [patient Y] ..."

The pathologist's suspicisions led to formal investigations by her company - part of the same group as Auckland's Labtests laboratory - by the DHB, the National Health Board and finally the commissioner.

Mr Hill said SCL breached the code of patient rights by using "unsafe practices" in its laboratory.

"In my view, SCL's processes for handling late-delivery breast samples such as Mrs X's included unsafe practices that directly contributed to Mrs X receiving biopsy results that did not belong to her."

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SCL's internal investigation found that the error was likely to have occurred when the biopsy samples were removed from their transport containers and placed into a plastic cassette used to hold the biopsy sample while in the processing machine. This would have been during the "cut-up" process, in which tissue samples are prepared for analysis.

A pathology registrar (trainee specialist) responsible for the transfer of samples into the cassettes told the commissioner's office, "I have been over this in my mind many, many times and I do not know when in the process or how the error was made."

Mr Hill concluded that "it is difficult to identify at which point in the cut-up process the error occurred".

"... the error was the result of a number of unsafe policies and practices in place at the laboratory at the time. Accordingly, I consider that the ultimate responsibility for the error must fall on the labatory itself."

Mr Hill criticised the DHB for how long it took to notify Mrs X of the mix-up.