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Background: Cancer antigen CA15-3 antigen is known as a valuable marker for the management of breast cancer. Methods: The analytical and clinical performance of the Access® BR Monitor Immunoassay System (Beckman Coulter) was evaluated at five different European sites and compared with a reference system, defined as CA15-3 on the Elecsys® System (Roche Diagnostics). Results: Total imprecision (%CV) of the BR Monitor ranged between 5.5% and 11.7%, and inter-laboratory reproducibility between 3.4% and 5.1%. Linearity upon dilution showed a mean recovery of 98.5% (SD±9.1%). Endogenous interferents had no influence on BR Monitor levels (mean recoveries: hemoglobin 112%, bilirubin 111%, triglycerides 108%). There was no high-dose hook effect up to 13,540 kU/L. Clinical performance investigated in sera from 1811 individuals showed a general correlation between the Access BR Monitor and Elecsys CA15-3 (R=0.797), with a slope of 1.383. CA15-3 serum levels, as measured by the BR Monitor, were low in healthy individuals (n=267, median=11.9 kU/L, 95th percentile=23.5 kU/L), higher in individuals with various benign diseases (n=549, medians=11.3–15.6 kU/L, 95th percentiles=21.6–54.6 kU/L) and even higher in individuals suffering from various cancers (n=995, medians=11.2–22.8 kU/L, 95th percentiles=30.0–429.7 kU/L). Best diagnostic accuracy for cancer detection against the relevant benign control group by the BR Monitor was found for locoregional and metastatic breast cancer, as well as for ovarian cancer [area under the curve (AUC) 0.619, 0.897 and 0.774]. Results for the reference CA15-3 assay were comparable (AUC 0.611, 0.887 and 0.818). Conclusions: The Access BR Monitor provides accurate methodological characteristics and demonstrates an analytical and clinical correlation with Elecsys CA15-3. Best diagnostic accuracy for the BR Monitor was found in breast and ovarian cancer. Our results also suggest a clinical value of the BR Monitor in other cancers. Clin Chem Lab Med 2008;46:612–22. [ABSTRACT FROM AUTHOR]