Multiplexed assays that can measure protein biomarkers and internal standards are highly desirable given the potential to reduce false positives and negatives. We report here the use of a chip-based platform that achieves multiplexed immunosensing of the ovarian cancer biomarker CA-125 without the need for covalent labeling or sandwich complexes. The sensor chips allow the straightforward comparison of detectors of different sizes, and we used this feature to scan the microscale size regime for the best sensor size and optimize the limit of detection exhibited down to 0.1 U/mL. The assay has a straightforward design, with readout being performed in a single step involving the introduction of a noncovalently attached redox reporter group. The detection system reported exhibits excellent specificity, with analysis of a specific cancer biomarker, CA-125, performed in human serum and whole blood. The multiplexing of the system allows the analysis of the biomarker to be performed in parallel with an abundant serum protein for internal calibration.