High incidence of macrotroponin I with a high-sensitivity troponin I assay.

BACKGROUND: Cardiac troponin is the preferred biomarker of myocardial injury. High-sensitivity troponin assays allow measurement of very low levels of troponin with excellent precision. After the introduction of a high-sensitivity troponin I assay the laboratory began to receive enquiries from clinicians about clinically discordant elevated troponin I results. This led to a systematic investigation and characterisation of the cause.
METHODS: Routine clinical samples were measured by the Architect High Sensitive Troponin-I (hsTnI) and the VITROS Troponin I ES assays (VitrosTnI). Results that were elevated according to the Architect but not the VITROS assay (Group 1) or results elevated by both assays but disproportionately higher on the Architect (Group 2) were re-analysed for hsTnI after re-centrifugation, multiple dilutions, incubation with heterophilic blocking reagents, polyethylene glycol (PEG) precipitation, and Protein A/G/L treatment. Sephacryl S-300 HR gel filtration chromatography (GFC) was performed on selected specimens.
RESULTS: A high molecular weight complex containing immunoreactive troponin I and immunoglobulin (macrotroponin I) was identified in 5% of patients with elevated hsTnI. Patients with both macrotroponin and myocardial injury had higher and longer elevation of hsTnI compared with VitrosTnI with peaks of both macrotroponin and free troponin I-C complex on GFC.
CONCLUSIONS: Circulating macrotroponin I (macroTnI) causes elevated hsTnI results with the Architect High Sensitive Troponin-I assay with the potential to be clinically misleading. The assay involved in this investigation may not be the only assay affected by macrotroponin. It is important for laboratories and clinicians to be aware of and develop processes to identify and manage specimens with elevated results due to macrotroponin.