The test is being developed at Llanelli's Prince Phillip Hospital with the help of Singleton Oncology and the University of Swansea.
During recent trials, nearly half of patients who were initially recommended for chemotherapy and endocrine therapy, had their treatment changed to endocrine therapy alone.
This test will have a major impact on the lives of many women, allowing them to get on with their lives following breast cancer surgery rather than having to go through four months of chemotherapy."
The 'Oncotype DX' test, which is only for women with early breast cancer, will give clinicians a clearer idea of the risk and benefit of chemotherapy.
More top news
Prince Charles was all smiles during his visit to Cardiff today meeting cricketers and enjoying a chat over a cup of tea.
The Prince of Wales met members of the England and Australia cricket teams ahead of the Ashes tournament which starts on Wednesday.
Summer seems like a dim and distant memory at the moment - in fact we'll have to wait until Thursday before the warm sunshine returns.