title: A practical assessment of the short-term in vitro stability of troponin I at the 99 th percentile.
abstract: 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.
authors.affiliation: Department of Clinical Biochemistry, Aberdeen Royal Infirmary, Aberdeen AB25 2ZD, UK. r.peake@nhs.net
authors.forename: Roy W A
authors.initials: RW
authors.lastname: Peake
authors.forename: Kevin A
authors.initials: KA
authors.lastname: Deans
authors.forename: Bernard L
authors.initials: BL
authors.lastname: Croal
mesh_terms: D006801:Humans; D009203:Myocardial Infarction / Q000097:blood / Q000175:diagnosis*; D055550:Protein Stability; D012016:Reference Values; D019210:Troponin I / Q000097:blood* / Q000737:chemistry*
chemical_list: D019210:Troponin I