Our Lady's Children's Hospital in Dublin is conducting a precautionary review of around 3,500 transcriptions of BRCA genetic test results, due to what it believes was a transcribing error with one test result.
The hospital said all the facts currently point to the case being an isolated incident caused by human error.
The patient has been diagnosed with ovarian cancer and is very ill.
"The Children's Hospital Group apologises to the woman at the centre of this transcription error and regrets the series of events that led to her current difficult situation.
"This fact has been communicated to the woman involved through her treating clinician," it said.
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The review is expected to be completed by next week.
Details of the case have been reported in the Ireland Edition of 'The Sunday Times' newspaper.
It is understood that the blood test for what is called the BRCA 1 gene was sent by Crumlin for testing in a UK hospital in September 2009.
The UK hospital reported a positive test for the gene and told Crumlin but this was not conveyed to the patient.
A positive result means a patient has a much higher risk of developing breast or ovarian cancer, than someone who does not have the BRCA gene mutation.
The hospital said it wanted to offer reassurance to other patients who have undergone testing that this is not a testing error and therefore there is no cause for concern or distress.
The issue involves the Department of Clinical Genetics at the hospital.
Our Lady's Children's Hospital said it cannot discuss individual patient cases.
The hospital review is expected to examine how the information was not correctly communicated to the woman by Crumlin.
Caoimhe Haughey, solicitor for the patient, said that her health was very much an evolving situation.
She said the patient was seriously ill and is focused on her treatment.
What she wanted was answers as to why this happened and an independent review.
Ms Haughey said that nothing had been communicated directly to the patient by Crumlin.
A spokeswoman for Minister for Health Simon Harris said the Department of Health was notified by the HSE of the case on Friday evening.
The Department has since briefed the Minister.
The Minister subsequently made contact with the Children's Hospital Group, which oversees Crumlin's operations.
"The Minister has been assured steps are being taken to ensure the error that occurred here is not replicated elsewhere."
The statement added: "The Minister has asked to be kept updated on these matters and the Hospital Group expects to be in a position to offer a clearer picture early next week."
Following queries, Crumlin Hospital told RTÉ News that it is committed to a culture that promotes an open and positive approach to incident management.
It said the hospital follows the HSE Incident Management Framework in relation to the reporting of incidents.
The hospital said that in July 2018, Crumlin provided extra resources to support the Department of Clinical Genetics to improve services.
Crumlin said that an external multi-professional team, led by an experienced health manager working in the UK, has been assisting the Department of Clinical Genetics to deliver an improved service for patients and clinical service users.