The test in question is called the Oncotype DX. Experts say it may not be accurate.
The Oncotype DX is a widely used breast cancer test that identifies a gene that is critical in determining which life-saving treatment a woman should get.
A study in the Journal of Clinical Oncology found the test results in a number of false-negatives for the HER-2 gene.
Women who test positive for the gene typically receive the targeted therapy Herceptin along with other medications to reduce the chances of recurrence and death.
Scientists say women could be getting the wrong treatment and that doctors and patients should not rely on this test alone.