Detection of myocardial injury in patients with unstable angina using a novel nanoparticle cardiac troponin I assay: Observations from the PROTECT-TIMI 30 Trial

BackgroundAt least 30% of patients with non–ST-elevation acute coronary syndrome present without evidence of myonecrosis using current generation troponin assays. A new generation of research assays for troponin that offer a >10-fold increase in analytical sensitivity has emerged.MethodsTo perform a pilot study to evaluate the clinical sensitivity of a new ultra-sensitive nanoparticle assay for cardiac troponin I (nano-cTnI), we identified 50 patients with unstable angina (serial negative cTnI) and 50 patients with non–ST-elevation myocardial infarction with an initially negative current generation cTnI result. We measured cTnI using an assay (Nanosphere, Northbrook, IL) that can detect pg/mL concentrations of cTnI (detection-limit 0.0002 μg/L).ResultsMeasured at 0, 2, and 8 hours with the nano-cTnI assay 44%, 62%, and 82% of patients with unstable angina defined by the current-generation assay had an elevated nano-cTnI result (≥0.003 μg/L, 99th percentile decision-limit, coefficient of variation <10%). In patients with definite myocardial injury (current-generation cTnI ≥0.1 μg/L) but an initially negative cTnI, 72% and 98% had a nano-cTnI ≥0.003 μg/L at 0 and 2 hours. No patient had a positive current-generation cTnI without an elevated nano-cTnI level.ConclusionsIn this pilot study using a nanoparticle assay for cTnI, myocardial injury was detectable in a substantial proportion of patients presently classified as having unstable angina, suggesting that ischemia with rest pain without injury is rare. The emergence of a new generation of troponin assays has the potential to lead to new clinical applications based on enhanced analytical performance at very low concentrations of troponin.