High sensitivity troponin I should only be measured in patients with a suspected acute coronary syndrome, in accordance with the trust Clinical Guideline CG62 High sensitivity (hs) troponin and suspected cardiac chest pain.
The use of high sensitivity troponin I measurements is clearly described in the Suspected cardiac chest pain pathway, which is Appendix 1 of CG62.
Troponin values must be used in the context of the patient clinical presentation. Serial sampling is recommended to detect the temporal rise and fall of troponin levels characteristic of AMI. The demonstration of a temporal rise and fall in troponin is needed to distinguish AMI from troponin elevations associated with non-AMI conditions, such as renal failure, arrhythmias, pulmonary embolism, chronic renal disease, myocarditis, and cardiotoxicity (please refer to Suspected cardiac chest pain pathway for specific cut-off and delta values).
Troponin is a protein complex which regulates the contraction of striated muscle. It consists of three subunits which are located periodically along the thin filament of the myofibrils. Troponin C binds calcium, troponin T attaches to tropomyosin on the thin filament, and troponin I inhibits actomyosin ATPase.
Troponin I (TnI), an inhibitory protein of the troponin-tropomyosin complex, exists in three distinct isoforms: cardiac muscle, slow-twitch skeletal muscle, and fast-twitch skeletal muscle. Each isoform is encoded by a distinct gene, each with a unique amino acid sequence, leading to 40% dissimilarity among isoforms. The cardiac form of troponin I is further unique having 31 additional amino acid residues on its N-terminal, not present in the skeletal forms, which allows for specific polyclonal and monoclonal antibody development. The cardiac specificity of this isoform improves the accuracy of diagnosis in patients with acute or chronic skeletal muscle injury and possible concomitant myocardial injury, and is the basis for its selection as a cardiac marker in the diagnosis of acute MI.
TnI is the only troponin isotope present in the myocardium and is not expressed during any developmental stage in skeletal muscle. TnI has a molecular weight of 24,000 daltons. Clinical studies have reported that TnI is released into the bloodstream within hours of the onset of symptoms of MI or ischemic damage.
The combined (male and female) 99 percentile value is 47ng/L (a high sensitivity troponin I assay by definition should be able to detect troponin I is greater than 50% of an apparently healthy population).
For the purposes of investigating suspected cardiac chest pain the following is reported with every result:
If baseline is less than 3ng/L OR baseline is less than 6ng/L AND delta 1 hour is less than 3ng/L, then Myocardial Infarction ruled out.
If baseline is greater or equal to 120ng/L OR delta 1 hour is equal or greater than 12ng/L then Myocardial Infarction is likely.
Results should always be interpreted in conjunction with the Suspected cardiac cheat pain pathway.
Serum is the preferred sample type.
Lithium heparin plasma may also be used, however any subsequent follow up specimens measured during the episode must also be lithium heparin.