In this work, an amperometric immunosensor for detection of myeloperoxidase (MPO) in human plasma is reported. Detection is based on the immobilization of anti-MPO antibodies onto magnetic beads (MBs). Following MPO immunocapture and washing steps, MBs are transferred to a customized modular detector device produced by 3D laser sintering. This tool integrates electrodes, electrical connectors, and a novel magnetic switch, whose functioning is founded on the vertical displacement of a permanent magnet. In this way, magnetic switching makes possible the confinement of MBs over the working electrode for electrochemical detection, followed by the release of MBs for electrode washing and reutilization. Notably, electrochemical detection is based on the endogenous MPO activity, which reduces reagent consumption and assay time compared to sandwich assays using enzyme-labeled antibodies. After optimization, the assay could be completed in 45 min and displayed a linear response between 0.9 and 60 ng mL-1 for MPO and a limit of detection of 0.4 ng mL-1. The real applicability of this approach is demonstrated by the ability to carry out the successful analysis of MPO in human plasma samples. Furthermore, the results allowed the classification of patients into three groups at risk of suffering cardiac events (i.e., low, medium, or high) and correlated well with data provided by a commercially available standardized method. © 2013 American Chemical Society.