Background Cardiac Troponin (cTn) is the biochemical gold standard for diagnosing myocardial infarction (MI). We compared the Siemens ADVIA Centaur high-sensitivity (hs-cTnI) assay with the Siemens Ultra assay (cTnI-U). Methods Over 3 months cTnI-U and hs-cTnI were measured simultaneously at Herlev-Gentofte Hospital. Acute myocardial injury was diagnosed using the 4th universal definition. Disputed cases were adjudicated using clinical data. We compared diagnostic accuracy using area under the curve (AUC) of the receiver operating characteristic. Outliers in between-assay differences were defined as a factor-5 difference and >= 1 measurement >40 ng/L. Patients with outlier differences were invited for re-sampling and tested with serial dilution and heterophilic blocking tubes. Results From the 18th January to the 20th April 2019, 4,369 samples on 2,658 patients were included. cTnI-U measured higher concentrations than hs-cTnI (mean 23%, -52-213%), resulting in a higher frequency of acute myocardial injury, 255 (9.6%) vs 203 (7.6%), p