Rapid immunoassays provide serum concentration levels of cTnI and cTnT that are approximately 96% sensitive and 94% specific for cardiac injury.
Each diagnostic company typically develops a unique antibody against troponin epitopes. There is only one assay available for cTnT. This initial cTnT assay was the only one patented and explains why there is one cTnT assay but many cTnI assays. Because cTnI has been developed by multiple diagnostic companies, there are several different antibodies that can be used to detect cTnI. Consequently, different assay methods may correlate with each other but will not harmonize to each other. That is, one cTnI test should trend or be linear with another cTnI assay but they will not likely give the same numeric values. Clinicians should never follow patients across two different troponin assays.
Reference ranges for troponin assays should use 99th percentile values. That is, any troponin value that is equal to or above the 99th percentile of a normal distribution of a healthy population should be used. If above this 99th percentile it should be considered positive. Troponin is similar in this way to drug screens; we are often less concerned with the number and more concerned with whether or not it crosses a particular diagnostic cutoff threshold.
Some typical cutoff concentrations for troponin are:
cTnT <0.01 ng/mL: No cardiac injury
cTnI< 0.03 ng/mL: No detectable cardiac injury
cTnI concentrations of 0.04-0.5 ng/mL are more significant for myocardial injury and even higher values (> 0.5 ng/mL) are more indicative of myocardial infarction.
