Study population

The present study investigated the clinical application of a contemporary sensitive troponin I assay in patients presenting with concomitant AF and suspected ACS. To describe the optimal usage of troponin I determination in such a setting, two independent prospective cohorts were used. First, an ACS registry was used as a derivation cohort to define optimal troponin I thresholds for identification of MI type 1 in patients with AF experiencing chest pain. Second, these specifically calculated diagnostic cut-offs were applied to a large, multicenter, real-world population of patients presenting with suspected ACS and AF as a validation cohort to test for the diagnostic performance compared with the standard 99th percentile troponin I cut-off. Only individuals of the two cohorts with available investigational troponin I measurements as well as available electrocardiogram on admission were used in the present post hoc analyses.

Derivation cohort

The Bad Nauheim ACS registry served as the derivation cohort. In this registry patients were enrolled consecutively from April 2003 until November 2006 and referred for early coronary angiography or primary percutaneous coronary intervention due to a potential ACS with an episode of chest pain within the prior 48 hours. Patients were either admitted directly by the emergency medical system or transferred from community hospitals. Medical history and data on the acute medical situation were assessed as described earlier [2]. Blood was drawn and an electrocardiogram was acquired upon admission directly before coronary angiography. All patients gave informed consent and the study was approved by the approval was obtained from the ethics board of the state of Hessen, Germany.

Validation cohort

A large, multicenter, all-comers study was used as a validation cohort. Patients were enrolled consecutively who presented with pain suspected to be due to ACS at the chest pain unit of the Johannes Gutenberg-University Medical Centre in Mainz, the Federal Armed Forces Hospital in Koblenz, or the University Hospital Hamburg-Eppendorf in Hamburg between January 2007 and December 2008. An electrocardiogram was acquired directly upon admission, and blood was drawn at admission and after 3 hours. Data on the acute medical situation and the patients’ medical history were assessed as described earlier [4]. Participation was voluntary. All patients provided written informed consent. The study was approved by local ethics committees in either Rheinland-Pfalz or Hamburg for all three centers.

Defining myocardial infarction and atrial fibrillation

The final diagnosis of MI was adjudicated according to the universal definition of MI [7, 8] in both cohorts as already described [2, 4]. Briefly, type 1 MI was diagnosed when there was evidence of myocardial necrosis that was consistent with myocardial ischemia together with clinical symptoms of ischemia or electrocardiographic changes indicative of new ischemia (new ST-segment or T-wave changes or new left bundle branch block) or imaging evidence of new loss of viable myocardium or detection of a culprit lesion on coronary angiography classified according to the Ambrose criteria. Myocardial necrosis was documented based on in-house troponin determination if there was at least one value above the cut-off value for 10 % imprecision of the respective conventional troponin test together with a rising or falling pattern (a change of at least 20 %) in serial in-house troponin measurements. The final diagnosis of MI type 1 was made by two independent cardiologists based on all available clinical, laboratory, and imaging findings blinded to the investigational troponin I measurements. In case of disagreement a third cardiologist was consulted. The presence of AF was determined in both cohorts based on the electrocardiogram obtained upon admission without differentiation of new onset of AF or persistent AF.

Laboratory measurements

Routine laboratory parameters, including creatinine, were measured immediately after blood withdrawal by standardized methods in both study cohorts. Additionally, venous blood samples were collected on admission and after 3 hours in the validation cohort, processed immediately, and stored at −80 °C until assay.

In-house troponin, represented by cardiac troponin T in the derivation cohort and two study centers of the validation cohort, was measured in serum using a conventional commercial one-step electrochemiluminescence immunoassay (cTnT, Elecsys 2010, Roche Diagnostics, Mannheim, Germany). The lower detection limit of this assay is 0.01 ng/mL, the 99th percentile is <0.01 ng/mL, and the lowest concentration measurable with a coefficient of variation (CV) <10 % is 0.03 ng/mL, which was used as the diagnostic cut-off. At the third study center of the validation cohort troponin I was used as in-house troponin for adjudication of final diagnosis. A conventional troponin I assay was used (Dimension RxL TnI, Siemens Healthcare Diagnostics, Erlangen, Germany) with lower detection limit of 0.040 ng/mL and measuring range of 0.04 to 40.0 ng/mL. The 99th percentile is 0.07 ng/mL, and the 10 % CV used as the diagnostic cut-off is 0.14 ng/mL.

As investigational troponin a contemporary sensitive troponin I assay (Architect STAT troponin I, Abbott Diagnostics) was measured in both cohorts. For this assay, the level of detection is 0.01 ng/mL with a measuring range of 0.01–50.0 ng/mL, and the 99th percentile and the lowest concentration with CV of 10 % is 0.032 ng/mL [17]. This investigational troponin I was measured by experienced technical assistants blinded to patient characteristics in stored frozen samples. Treating physicians and research staff involved in enrollment of study participants were unaware of the measured investigational troponin I values.

Statistical analyses

Continuous skewed variables are described as median and interquartile range and symmetric variables are presented as mean with standard deviation. Receiver operating characteristic (ROC) curves based on continuous troponin I levels were calculated in both cohorts.

In the derivation cohort of 90 patients with AF, optimized thresholds were computed by determining the cut-offs that maximized i) the sum of specificity and sensitivity (Youden-optimized cut-off, named “unweighted”) and those that yielded ii) 90 % sensitivity and iii) 90 % specificity, respectively. In addition we have considered the 99th percentile of the assay as cut-off. The uncertainty of choices i) – iii) is reflected by 95 % confidence intervals that were obtained nonparametrically by taking the 2.5 % and 97.5 % percentiles from 2,000 bootstrap replications of these evaluations.

These cut-offs have been applied to the validation cohort of 314 patients. Sensitivity, specificity, positive predictive values (PPVs), and negative predictive values (NPVs) for the individual patient groups were calculated by applying the different troponin I cut-off values and consecutively calculating the corresponding values from a two-by-two factorial design. Corresponding confidence intervals for all these proportions were calculated according to Clopper-Pearson.

Relative and absolute changes in the concentration between admission and after 3 hours (that is, absolute differences and differences divided by the baseline value times 100 %) in the validation cohort were regarded as new biomarkers that gave rise to analogously defined cut-offs. Empirical kernel density estimations of these absolute and relative changes have been plotted for both subgroups, MI and non-MI patients (Fig. 2), where the bandwidths have been chosen so as to provide optimal insight into the qualitative distribution of values.

A one-sided P < 0.05 was considered significant. All analyses were carried out using R 2.15 and 3.1.1 (R Foundation for Statistical Computing, Vienna, Austria).