Background: Provista Diagnostics has developed a test that analyzes serum concentrations of 5 protein biomarkers in order to detect breast cancer. The dtectDx Breast test utilizes a proprietary algorithm that has been described previously (Weber et al. 2010). In this study, it was noted that the algorithm performs best in women under age 50. The aim of this study was to evaluate the performance characteristics of dtectDx Breast in women under age 50 in a commercial setting and compare the results with data from the previous clinical study. Methods: The dtectDx Breast test measures the concentrations of IL-8, IL-12, VEGF, CEA, and HGF via ELISA. These data combined with select patient characteristics and Provista’s proprietary algorithm result in a test value that is characterized as normal or elevated. dtectDx Breast test reports issued for women under age 50 were reviewed from a 3-year time period and prescribing physicians were interviewed regarding follow-up care and outcome measures (largely imaging studies, if warranted). Results: Of the 908 patients, 8 samples were rejected based on serum quality. Of the remaining 900 patients, 121 were reported as elevated (12.7%). In 4 cases, these elevated results were confirmed cases of breast cancer. Of these, 2 patients initially showed no screening evidence of cancer, but upon further evaluation (after receipt of dtectDx Breast results) were diagnosed with breast cancer. dtectDx correctly identified DCIS 66% of the time (n=2). Conclusions: These results describe the use of dtectDx Breast in a clinical setting and confirm that the assay behaves similarly to previously published results (Weber et al 2010). While the false-positive rate is higher than predicted (12.7% vs 6.8%), the assay correctly identified 4 of 4 invasive cancers and 2 of 3 DCIS cases. Since two of the invasive cancer cases were originally not detected via standard screening procedures, the assay has demonstrated important clinical utility when used in conjunction with mammography/standard of care. Here we show that, in the commercial patient population, when combined with standard of care, dtectDx Breast improves the detection of breast cancer in women under 50.