Troponins

Cardiac troponins (cTn) are muscle regulatory proteins that control the calcium-mediated interaction of actin and myosin (muscle contraction) and consist of cytosolic and structural pools, with most troponin present as structural proteins [25]. Cardiac troponin I (cTnI) and cardiac troponin T (cTnT) are the two protein subunits of the troponin tri protein-complex (troponin C being the third but is not cardiac specific), which are actually measured in plasma to assess cardiac damage. Both cTnI and TnT are independently measured using monoclonal antibodies in immunometric assay formats resulting in clinical assays specific (almost exclusively) to cardiac muscle damage. While they do not specifically identify the underlying cause of cell injury, they are considered to be the current gold-standard biomarker for myocardial injury and necrosis [13].

Cardiac troponin concentrations typically begin to rise 2–3 h after the onset of acute MI. At 2–3 h after initial presentation, up to 80% of patients with MI will have detectable troponin elevations. Markers that rise earlier than troponins, such as myoglobin and CK isoforms, actually have been shown to provide little additional diagnostic or clinical utility when a sufficiently analytically sensitive troponin assay (i.e., an assay with a coefficient of variation approaching 10% at the 99th percentile) is employed in the lab [26], although, in practice, desired precision at the 99th percentile is typically not achieved for existing (contemporary) US Food and Drug Administration (FDA) approved troponin assays that are in use. cTnI is considered specific to the heart muscle as no other isoform of this fraction has even been detected or discovered in other muscle tissue [27]; however, cTnT is believed to be potentially expressed to a very minor extent in the skeletal muscle, due to its previous measurement in patients with specific skeletal muscle myopathies [28]. TnI can also be mildly overexpressed in these rare patients, but the prevalence of elevated expression compared to TnT is much lower. Despite their current widespread use, contemporary cardiac troponin assays lack sex-specific reference value reporting, even for widely used commercial assays that indicate 99th percentile cutoffs or ranges 1.2–2.4-fold higher in males than females [29]. Healthy animal data demonstrates higher circulating cTnI concentrations in males compared to females of the same species [30].

Further investigation has evaluated sex differences in troponin and clinical manifestations of CVD. Säfström et al. evaluated exercise stress testing and cTnT in subjects with suspected myocardial ischemia [31]. The women studied were older and had higher incidence of CVD-related events and diagnoses; however, only 49% of the women compared to 69% of men had cTnT levels that met the threshold for MI (≥ 0.20 μg/l) [31]. Shoaibi and colleagues evaluated women and men diagnosed with AMI using standard biomarker criteria and found no variation in the assay sensitivity and specificity or troponin level by sex [32]; as well as gender, assay performance and sensitivity influenced outcome [33]. Newer “high sensitivity” clinical and preclinical cTnI and cTnT assays that do not demonstrate loss of specificity could be helpful in closing the gender bias. These new high-sensitivity assays will be expected to demonstrate robust precision and sensitivity, e.g., a highly reproducible CV < 10% at the 99th percentile concentration of the reference population, that can also be reproduced in routine practice in the clinical lab. Another important change with high-sensitivity assays will be the reporting units going to nanograms/liter instead of nanograms/milliliter (as currently reported); this is due to the at least threefold increase in sensitivity that is gained with high-sensitivity troponin assays over existing FDA-approved assays. Thus far in Europe, most high-sensitivity assays have been establishing sex-specific reference range criteria.

Overall, these data suggest that at-risk women may be missed when using male sex-specific thresholds and that, as a result, those women who meet standard MI troponin measurement criteria have suffered a greater degree of myocardial damage [33]. Indeed, a recent study demonstrated that standard troponin criteria failed to detect one out of five acute MIs occurring in women, which was, not surprisingly, associated with an elevated death rate [34], while another study demonstrated varying 99th percentiles for hs-cTnI related to lack of a uniform protocol for healthy reference population selection [35].