Background: High-sensitivity cardiac troponin T (hs-cTnT) blood concentrationswere shown to exhibit a diurnal rhythm, characterized by gradually decreasing concentrations throughout daytime, rising concentrations during nighttime and peak concentrations in the morning. We aimed to investigate whether this also applies to (h)s-cTnI assays and whether it would affect diagnostic accuracy for acute myocardial infarction (AMI). Methods: Blood concentrations of cTnI were measured at presentation and after 1 h using four different cTnI assays: three commonly used sensitive (s-cTnI Architect, Ultra and Accu) and one experimental high-sensitivity assay (hs-cTnI Accu) in a prospective multicenter diagnostic study of patients presenting to the emergency department with suspected AMI. These concentrations and their diagnostic accuracy for AMI (quantified by the area under the curve (AUC)) were compared between morning (11 p.m. to 2 p.m.) and evening (2 p.m. to 11 p. m.) presenters. Results: Among 2601 patients, AMI was the final diagnosis in 17.6% of patients. Concentrations of (h) s-cTnI as measured using all four assayswere comparable in patients presenting in themorning versus patients presenting in the evening. Diagnostic accuracy for AMI of all four (h) s-cTnI assays were high and comparable between patients presenting in the morning versus presenting in the evening (AUC at presentation: 0.90 vs 0.93 for s-cTnI Architect; 0.91 vs 0.94 for s-cTnI Ultra; 0.89 vs 0.94 for s-cTnI Accu; 0.91 vs 0.94 for hs-cTnI Accu). Conclusions: Cardiac TnI does not seemto express a diurnal rhythm. Diagnostic accuracy for AMI is very high and does not differ with time of presentation. Clinical trial registration: NCT00470587, http://clinicaltrials.gov/show/NCT00470587 (c) 2018 Elsevier B.V. All rights reserved.