Background: Serial troponin measurement is important for the diagnosis of myocardial infarction. As troponin concentrations approach the cut-off for detectable myocardial necrosis, smaller changes in troponin as a result of decreased in vitro stability may be sufficient to generate changes of apparent clinical significance. This is particularly relevant regarding retrospective 'add-on' testing on samples several hours after venepuncture. We investigated the stability of troponin I (cTnI) at values close to the 99th percentile limit.
Methods: Serum samples with baseline cTnI concentrations in the range 0.04-0.15 μg/L were analysed routinely using the Siemens TnI-Ultra assay. Follow-up analysis was carried out at 6, 9, 12, 24 and 48 h post-venepuncture after storage at room temperature and in the cold room.
Results: There was a significant decrease in cTnI concentration after 6 h post-venepuncture compared with baseline levels (P < 0.001). The maximum percentage change was -17.1% observed after 48 h storage at room temperature. Of samples with baseline cTnI concentration 0.040 μg/L, 80% had a cTnI concentration below 0.040 μg/L on re-analysis after 6 h.
Conclusions: Requests for retrospective addition of troponin measurement on samples several hours after venepuncture is commonplace in many laboratories. Analysis of samples drawn >6 h previously may produce values below the cut-off for myocardial necrosis (0.040 μg/L) that would have been detectable if measured earlier as a result of decreased in vitro stability. Significant percentage decreases in cTnI concentration following storage may also have implications in lowering the threshold for an apparently clinically significant change to occur.