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
Gwent Police is appealing for information and calling for extra vigilance after a number of distraction incidents across the area.
Beaumaris Castle on Anglesey will finally be 'completed' over 700 years after it was built, thanks to a £170,000 investment from Cadw.
Residents have been warned to expect 'explosions and blank gunfire' during large scale anti-terror exercises taking place across Wales,