Because the results of ER and PR testing can make a difference in a patient's treatment and chance of recurrence, it's important that these tests are accurate. This guideline was developed to help both doctors and laboratories know how to improve the accuracy of ER and PR testing for patients with breast cancer. Understanding the ER/PR status of the primary tumor and any recurrent tumors can help doctors make sure that patients receive the appropriate treatment and avoid side effects of a treatment that may not work. Use this guideline to talk with your doctor about the accuracy of your ER and PR test results and what that means for your treatment.