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
Any heavy showers will clear away northeastwards through late evening.
The Scarlets were comprehensively beaten 38-16 by Leinster in Dublin.
Cardiff Blues survived an onslaught from Pau's powerful pack to book a place in the European Challenge Cup final.