Automated Immunoassay of Cardiac Troponin I in Serum Evaluated

We evaluated the Opus Troponin I assay (Behring Diagnostics, Westwood, MA), a two-site sandwich, fluorogenic ELISA that uses two goat polyclonal antibodies directed against different protein segments unique to cardiac troponin I (cTnI) (1). Pipetting, incubations, measurements, and data-reduction steps are performed on the Opus analyzer; the first test result requires 20 min. The assay measures concentrations of cTnI in serum as great as ∼135 μg/L. The calibration appeared to be stable for at least 4 weeks. Serial dilution of a human serum sample with a high concentration of cTnI showed no significant curvature when the curve obtained was tested for linearity [quadratic regression: y = −0.25 + 96.1x + 6.31x2, with the coefficient of x2 not significantly different from 0 (P = 0.27)] (2). Linear regression analysis of these data confirmed the high linearity of the response (r = 0.9998). The minimum detectable cTnI concentration, assessed by 10 replicate measurements of a human serum containing no detectable cTnI concentration and defined as the cTnI value corresponding to the fluorescence signal 3 SD greater than the mean found for this serum, was estimated as 0.38 μg/L. The Opus analyzer, however, reports results <0.50 μg/L as “<0.5 μg/L.”