The measurement of cardiac troponin I (cTnI)1 has become the gold standard for the clinical diagnosis of myocardial infarction (1). Varieties of commercial sandwich-type immunoassays are used for the measurement of cTnI concentration in human serum or plasma. Several of these assays use pairs of monoclonal antihuman cTnI antibodies with high specificities for the invariant part of the cTnI molecule (amino acid residues 30–110), in accordance with the recommended guidelines (2). These assays are often based on magnetic beads or latex particles to take advantage of the ease of bead/particle washing, which can minimize assay interference by matrix proteins. Signal amplification by means of chemiluminescence or electrochemiluminescence allows the detection of serum cTnI at concentrations <0.05 μg/L. As a part of the recent effort of the IFCC Working Group for Standardization of Troponin I to develop a cTnI reference measurement system (1), we have used various measurement techniques to investigate binding affinities between 6 monoclonal antibodies (mAbs) obtained from HyTest (each serving as either the capture or the detection antibody) and cTnI, in the form of either a reference material (NIST SRM 2921) or a cTnI-positive serum pool (PS). One of the techniques is the multiplexed bead array (3), which resembles the commercial bead- or particle-based immunoassays. The capture mAb molecules are covalently immobilized on different bead populations to capture cTnI. The bound cTnI …