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Cardiac troponin (cTn) is a highly specific biomarker of myocardial injury and is central to the diagnosis of acute myocardial infarction (AMI). By itself, however, cTn cannot identify the cause of myocardial injury. ‘Troponinitis’ is the condition that leads clinicians to falsely assign a diagnosis of AMI based only on the fact that a patient has an elevated cTn concentration. There are many causes of myocardial injury other than AMI. Clinicians are required to differentiate myocardial injury caused by AMI from other causes.In part 1 of this series on cTn, we provide a structured overview to help practising clinicians to interpret ‘positive’ cTn results appropriately. There are three core principles. First, when reviewing a cTn result, clinicians must carefully consider the clinical context. Only this can distinguish primary (termed type 1) AMI caused by coronary artery disease from secondary (termed type 2) AMI caused by another condition with an imbalance in the supply and demand of oxygen to the myocardium. Second, clinicians must consider the patient’s baseline condition in order to determine the presence or absence of factors that may predict a chronic cTn elevation. Third, clinicians should routinely use serial sampling to detect a change of cTn that is expected in patients with acute (rather than chronic) myocardial injury. Using these simple principles, clinicians can avoid underdiagnosis and overdiagnosis of AMI.