The Antibody Configurations of Cardiac Troponin I Assays May Determine Their Clinical Performance Stefan James, 1,3 Mats Flodin, 2 Nina Johnston, 1 Bertil Lindahl, 1,3 and Per Venge 2* Background: Previous studies have shown superior clinical performance of the cardiac troponin I (cTnI) assay from Beckman-Coulter Diagnostics. This assay had a unique combination of monoclonal antibodies with 2 monoclonal antibodies directed against epitopes near the NH 2 terminus of the heart-specific region of troponin I. The approach has been adopted by the new cTnI assay from Abbott Diagnostics. The aim of our study was to investigate whether this approach affects the clinical performance of cTnI assays. Methods: Cardiac troponin concentrations were mea- sured in a random sample of patients with unstable coronary artery disease included in the GUSTO IV trial (n  696) by the AccuTnI (Beckman-Coulter Diagnos- tics), Architect cTnI (Abbott Diagnostics), Immulite 2500 cTnI (Diagnostics Products Corporation), and Elecsys 2010 cTnT (Roche Diagnostics) assays and related to the 1-year mortality. The primary cutoff concentrations were based on the 99th percentile upper reference limits and an imprecision (CV) <10%. Results: The sensitivities of the AccuTnI and Architect cTnI assays in identifying patients who died within 1 year were equal and were significantly higher (P <0.05) than those of the Immulite 2500 cTnI and the Elecsys cTnT assays. The concordance between the AccuTnI and Architect cTnI assays was 97%, but concordances be- tween the Architect cTnI and the Elecsys cTnT assays were 89%–92% with more at-risk patients (P <0.01 to P <0.001) identified by the Architect cTnI assay. Conclusions: The Architect cTnI assay has clinical per- formance similar to that of the AccuTnI, probably as a result of the inclusion of a monoclonal antibody against troponin I epitope 41– 49 in the assay. © 2006 American Association for Clinical Chemistry Modern assays of cardiac troponins specifically reflect the leakage of proteins from myocardial cells (1–3 ), and any increase of troponin concentrations in the blood seems to be a signal of a poor condition of the myocardium and is related to an unfavorable outcome of the patient (4–9). In a recent report we showed that some assays had superior clinical performance compared with other troponin as- says with similar analytic sensitivities and performances (10, 11); the cardiac troponin I (cTnI) 4 assay (AccuTnI) from Beckman Coulter (Fullerton, CA) identified 10% more patients with an adverse outcome than the AxSYM assay (Abbott, Abbott Park, IL) and the Liaison assay (Diasorin, formerly Byk-Sangtec). In addition, compared with the troponin T assay of Roche, the AccuTnI assay was superior. The choice of antibodies in the cTnI assays was different. The AccuTnI assay is based on a pair of monoclonal antibodies lying next to each other and di- rected against epitopes in the heart-specific and the stable region of the molecule close to the NH 2 terminus (epitopes 24 – 40 and 41– 49), whereas the 2 other assays incorporate one antibody directed toward the heart-spe- cific region close to the NH 2 terminus (i.e., in the region of epitopes 20-39) and the other antibody directed against epitopes in the stable part closer to the COOH terminus (epitopes 87–91 and 80 –110, respectively). With the accu- mulating knowledge of the presence in blood of many different forms of troponins and interfering factors in the form of autoantibodies (12–14 ), we postulated that the assay configuration with respect to epitope specificity of 1 Department of Medical Sciences, Cardiology, and 2 Department of Med- ical Sciences, Clinical Chemistry, Uppsala University, Uppsala, Sweden. 3 Uppsala Clinical Research Centre, Uppsala, Sweden. * Address correspondence to this author at: Department of Clinical Chem- istry and Pharmacology, University Hospital, SE-751 85 Uppsala, Sweden. Fax 46-186113703; e-mail per.venge@akademiska.se. Received December 5, 2005; accepted February 17, 2006. Previously published online at DOI: 10.1373/clinchem.2005.064857 4 Nonstandard abbreviations: cTnI and cTnT, cardiac troponin I and T, respectively; GUSTO IV, Global Utilization of Strategies to Open occluded arteries IV; DPC, Diagnostics Products Corporation; URL, upper reference limit; SWISCH, Sweden, Women and Men and Ischemic Heart Disease; and FRISC II, Fast Revascularisation during InStability in CAD trial. Clinical Chemistry 52:5 832– 837 (2006) Proteomics and Protein Markers 832