Research by the University of Southampton has raised important questions about the applicability of high-sensitivity cardiac troponin I (hs-cTnI) as an arbiter of type 1 acute myocardial infarction (MI) in patients who do not have a history consistent with the diagnosis.
The prospective, observational cohort study, published in the BMJ, found that the upper limit of normal (ULN) for hs-cTnI varies significantly according to the clinical setting, age and sex of the patient, and the location when the biochemistry test was requested.
The study included 20,000 consecutive inpatients and outpatients at University Hospital Southampton undergoing blood tests for any clinical reason. Hs-cTnI concentrations were measured in all study participants and nested for analysis except when the supervising doctor had requested hs-cTnI for clinical reasons. The manufacturer-recommended 99th centile was used to determine ULN.
The data revealed that the 99th centile of hs-cTnI for the whole population was 296 ng/L compared with the manufacturer’s quoted level of 40 ng/L. Hs-cTnI concentrations were greater than 40 ng/L in one in 20 (5.4%) of the total population.
After excluding participants diagnosed with MI and those in whom hs-cTnI was requested for clinical reasons, the 99th centile was 189 ng/L. This rose to 563 ng/L for inpatients and 65 ng/L for outpatients.
Patients from the emergency department had a 99th centile of 215 ng/L, with 6.07 per cent having levels above the recommended ULN. In critical care units, 39.02 per cent of all patients and 14.16 per cent of all medical inpatients had an hs-cTnI concentrations greater than the recommended ULN.
The study authors say the findings highlight the need for clinical staff to interpret hs-cTnI concentrations carefully and systematically when making a diagnosis of acute MI, particularly type 1 MI.