Biochemical differences between cTnT and cTnI and their significance for diagnosis of acute coronary syndromes.

Although assays for cardiac troponin T (cTnT) and cardiac troponin I (cTnI) exhibit similar clinical performance in patients with acute coronary syndromes for diagnosis and risk stratification, there are differences in the release and clearance of these proteins from damaged myocytes. There are also differences in the percentages of patients with chronic renal failure and skeletal muscle myopathies that have abnormal results. A review of the biochemistry of these proteins may provide insights as to the nature of these differences. Cardiac troponin T has a higher overall tissue concentration and free cytoplasmic concentrations than cTnI, and appears in blood of patients with acute myocardial infarction as a mixture of complexed (cTnT-I-C) and free cTnT. Cardiac troponin I is more hydrophobic and appears in blood predominately in the binary complex (I-C) form, with smaller amounts of the ternary (T-I-C) complex. Degradation of the complex will lead to faster clearance of the protein. There are also differences in standardization and availability of commercial troponin kits. Cardiac troponin T assays are available from only one manufacturer and are standardized to a single material, whereas cTnI is available from multiple vendors. While there is some degree of correlation between assays, there are significant proportional biases between results which can be problematical when different tests are made on the same patient.