Standardisation of cardiac troponin I measurement: past and present

The laboratory measurement of cardiac troponin (cTn) concentration is a critical tool in the diagnosis of acute myocardial infarction (MI). Current cTnI assays produce different absolute troponin numbers and use different clinical cut-off values; hence cTnI values cannot be interchanged, with consequent confusion for clinicians. A recent Australian study compared patient results for seven cTnI assays and showed that between-method variation was approximately 2- to 5-fold. A major reason for poor method agreement is the lack of a suitable common reference material for the calibration of cTnI assays by manufacturers. Purified complexed troponin material lacks adequate commutability for all assays; hence a serum-based secondary reference material is required for cTnI with value assignment by a higher order reference measurement procedure. There is considerable debate about how best to achieve comparability of results for heterogeneous analytes such as cTnI, whether it should be via the harmonisation or the standardisation process. Whereas harmonisation depends upon consensus value assignment and uses those commercial methods which give the closest agreement at the time, standardisation comes closer to the true value through a reference measurement system that is based upon long-term calibration traceability. The current paper describes standardisation efforts by the International Federation of Clinical Chemistry and Laboratory Medicine Working Group on Standardization of cTnI (IFCC WG-TNI) to establish a reference immunoassay measurement procedure for cTnI of a higher order than current commercial immunoassay methods and a commutable secondary reference material for cTnI to which companies can reference their calibration materials.