Cardiac biomarkers of acute coronary syndrome: from history to high-sensitivity cardiac troponin

A rapid and accurate diagnosis is critical in patients with presumed acute coronary syndrome for the initiation of effective evidence-based medical management and revascularization. The third universal definition of myocardial infarction defines an acute myocardial infarction (AMI) as evidence of myocardial necrosis in a patient with the clinical features of acute myocardial ischemia, and defines the 99th percentile of cardiac troponins as the decision value for AMI [1]. Clinical assessment, 12-lead ECG and cardiac troponin (cTn) I or T form the diagnostic cornerstones of patients with acute onset chest pain. Contemporary sensitive and high-sensitivity cardiac troponin (hs-cTn) assays have increased diagnostic accuracy in patients with acute chest pain in comparison with conventional cardiac biomarkers [2]. Rapid rule-in and rule-out diagnostic strategies for patients with chest pain in the emergency department (ED) are now available, and help clinicians to risk stratify patients and enable discharge of those deemed to be at very low risk. In principle, this improves assessment and makes ED care more cost effective. However, there is potential for confusion, misuse and unnecessary follow-up examinations when they are used imprudently [3]. Novel hs-cTn assays are able to quantify troponin in the majority of healthy individuals [4]. Although hs-cTn assays are very sensitive, they are less specific for AMI when using the 99th percentile as a single cutoff level [5]. Even when a troponin rise is consistent with a diagnosis of AMI, other cardiac diseases such as myocarditis, Tako-tsubo cardiomyopathy or shock can produce significant changes of troponin as well. Interpretation of the results is heavily dependent on the clinical context in which it is requested.

The aim of the current article is to review the history and evolution of cardiac biomarkers of acute coronary syndrome, define what troponin is, and aid in the use of the hs-cTn in clinical practice according to current guidelines.

Aspartate Transaminase (AST) became the first biomarker used in the diagnosis of AMI [6] (Fig. 1). AST was widely used in the 1960s and was incorporated into the World Health Organization (WHO) definition of AMI [7]. However, AST is not specific for cardiac muscle, and its detection is, therefore, not specific for cardiac damage. By 1970s, two further cardiac biomarkers were in use: lactate dehydrogenase (LDH) and creatine kinase (CK). Although neither is absolutely specific for cardiac muscle, CK is more specific than LDH in the context of AMI, especially in patients having other co-morbidities such as muscle or hepatic disease [8]. Myoglobin is a small globular oxygen-carrying protein found in heart and striated skeletal muscle [9]. The first method to detect myoglobin in the serum was developed in 1978. Myoglobin rises after acute myocardial injury, and it became a useful cardiac biomarker in the differential diagnosis of suspected AMI [10]. In the era of hs-cTn, testing of myoglobin as an early marker of myocardial necrosis is no longer recommended, neither as a single marker nor in a multi-marker strategy [11, 12]. Eventually, advancements in electrophoresis allowed the detection of cardiac-specific iso-enzymes of CK and LDH, i.e., CK-MB and LDH 1 + 2 [13]. Cardiac muscle has higher CK-MB levels (25–30%) compared to skeletal muscle (1%), which is mostly composed of CK-MM. These assays played an important role in the diagnosis of AMI for two decades, and were included as one of the diagnostic criteria to rule out AMI by the WHO in 1979 [14]. However, the lack of specificity and the high rate of false-positive results limited their usefulness. A more cardiac-specific biomarker was needed.

In 1965, a new protein constituent of the cardiac myofibrillar apparatus was discovered, which subsequently came to be known as troponin [15]. In the late 1990s, a sensitive and reliable radioimmunoassay was developed to detect serum troponin [16]. Numerous studies demonstrate that troponins appear in the serum 4–10 h after the onset of AMI [17]. Troponin levels peak at 12–48 h, but remain elevated for 4–10 days. The sensitivity for detecting troponin T and I approaches 100% when sampled 6–12 h after acute chest pain onset [18]. Therefore, in the context of acute chest pain, to reliably rule out AMI, patients need to have a repeat troponin sample 6–12 h after the initial assessment. Consequently, patients were increasingly admitted to observational chest pain units.

Troponin is a component of the contractile apparatus within skeletal and cardiac myocytes. Along with calcium ions, troponin proteins regulate and facilitate the interaction between actin and myosin filaments as part of the sliding filament mechanism of muscle contraction. Cardiac troponin (cTn) is a complex comprising three subunits:

Troponin C is synthesised in skeletal and cardiac muscle. Troponin T and I isoforms are highly specific and sensitive to cardiac myocytes and, therefore, are known as cardiac troponins (cTn). The detection of cTn-T or cTn-I in the blood stream is, therefore, a highly specific marker for cardiac damage [19]. 92–95% of troponin is attached to the actin thin filaments in the cardiac sarcomere, and the remaining 5–8% is free in the myocyte cytoplasm [20]. Free, unbound cTn constitutes the ‘early releasable troponin pool’ (ERTP) [4]. The concept of the ERTP helps when considering the various mechanisms of troponin release into the blood stream. ERTP is thought to be released immediately following myocyte injury and, assuming normal renal function, this would be cleared promptly. This is contrary to the structurally bound cTn, which degrades over a period of several days causing a more stable and gradual troponin release.

The plasma half-life of cTn is around 2 h. Although the precise mechanism by which troponin is eliminated from the body remains unclear, it is hypothesized that troponin is cleared, at least in part, by the renal reticulo-endothelial system [21].

Most hospitals now have replaced conventional cTn tests with the new 5th generation hs-cTn T and I assays which can detect troponin at concentrations 10- to 100-fold lower than conventional assays (Fig. 2). Various terms for “more sensitive” cTn assays have been proposed for marketing purposes. Troponin assays are recommended to be differentiated in conventional, sensitive and high-sensitivity cTn assays. Basically, hs-cTn assays detect troponin with higher sensitivity and precision at an earlier point of time [22], and allow detection and quantification in 50% to ideally 95% of healthy individuals (Table 1) [23]. Troponins are quantitative markers of cardiomyocyte injury, and the likelihood of AMI increases with increase in the level of cTn [24]. The negative predictive value (NPV) of hs-cTn assays is >95% for AMI exclusion when patients are tested on arrival at the ED [25]. If this is repeated at 3 h, this rises to nearly 100% [26]. Shah et al. demonstrated that using lower cutoffs for hs-cTn I (5 ng/L) identifies low-risk patients for the composite outcome of index myocardial infarction, and myocardial infarction or cardiac death at 30 days with an NPV of 99.6% (95% CI 99.3–99.8%) [27]. A recent systematic review and meta-analysis demonstrated that ‘lower cutoffs’ (3–5 ng/L versus 14 ng/L) for a single baseline hs-cTn T measurement improve sensitivity for AMI markedly, and can be used as a rule-out test in patients presenting more than 3 h after symptom onset [28]. Therefore, hs-cTn facilitates earlier exclusion of AMI, contributing to reduced ED length of stay, and earlier treatment for AMI resulting in improved outcomes [29]. However, the high sensitivity of these assays may result in increased numbers of patients with elevated hs-cTn levels being admitted for further assessment.

There is a misconception that troponin elevation is secondary only to myocyte injury and necrosis. There are six mechanisms that have been proposed to explain the release of troponin into the bloodstream: normal cell turnover, myocyte necrosis, apoptosis or programmed cell death, proteolytic fragmentation, increased cell membrane permeability and membranous blebs.

Whether or not cTn is detectable, or even elevated, is therefore dependent on the balance of a host of interdependent factors, including the sensitivity of the test. Furthermore, there are potentially other, as yet not described mechanisms involved in the release of cTn. For example, it is still unknown why cTn is elevated in certain extra-cardiac disease processes such as sepsis. Whether ischemia causes elevated cTn in the absence of myocyte necrosis remains controversial [30]. Some animal and human studies demonstrate an association between reversible ischemia (no evidence of MI) and cTn elevation [31], whereas others fail to do so [32]. Increased myocardial strain is also considered to be associated with cTn elevation [33].

There is a risk of misinterpretation of elevated troponin results. Almost 13% of patients presenting with raised hs-cTn and chest pain eventually prove not to have ACS [34]. Hs-cTn can be detected in patients with various cardiac and non-coronary cardiovascular co-morbidities (Fig. 3).

More sensitive cTn assays also maintain high diagnostic accuracy in patients with renal dysfunction when assay-specific higher optimal cutoff levels are used [35]. Currently, there is no consensus on whether diagnostic criteria for AMI should differ for patients with and without impaired renal function [36]. The high prevalence of persistently elevated more sensitive cTn levels in patients with chronic kidney disease (CKD) cannot primarily be explained by reduced renal clearance alone [36]. The etiology of persistent troponin elevation in CKD remains incompletely explained and controversial. The underlying process appears to be multifactorial related to both increased subclinical cardiac damage (uremic toxicity, ischemic heart disease, heart failure or hypertensive heart disease) and decreased renal clearance in this population [37, 38]. The predictive value of cTn assays is maintained in patients with CKD [39]. Troponin elevation in patients with CKD should thus be taken seriously, and not merely be discounted as the result of decreased renal clearance.

Cardiac biomarkers for diagnosis of AMI have become more and more sensitive in recent decades. The currently used hs-cTn assays are highly valuable for rule-in and rule-out of AMI. International guidelines have been published for appropriate use of hs-cTn. Acute changes in hs-cTn complement the quantitative information provided by hs-cTn, and help in the differential diagnosis of diseases with chronic, stable troponin elevations vs. diseases with acute troponin elevations and acute cardiac damage. Serial testing of hs-cTn does not differentiate Type 1 from Type 2 myocardial infarction and, hence, integrating the results of hs-cTn measurements with robust clinical assessment remains the optimal approach.