Introduction

According to the European Society of Cardiology, American College of Cardiology Foundation, American Heart Association, and World Health Federation Expert consensus document on the third universal definition of myocardial infarction, acute myocardial infarction can be diagnosed in several ways, one of which depends on cardiac enzymes. [1][2][3][4][5] The pertinent definition is:

"Detection of a rise and/or fall of cardiac biomarker values (preferably cardiac troponin) with at least one value above the 99 percentile upper reference limit and with at least one of the following: 

• Symptoms of ischemia
• New or presumed new significant ST segment-T wave changes or new left bundle branch block
• Development of pathological Q waves on ECG
• Imaging evidence of a new loss of viable myocardium or new regional wall motion abnormality
• Identification of an intracoronary thrombus by angiography or autopsy."

The morbidity and mortality associated with acute myocardial infarction are well understood and discussed elsewhere. Given the known morbidity and mortality associated with acute myocardial infarction and the importance of early diagnosis and management, the above definition places a heavy burden on cardiac enzymes as their elevation alone, along with symptoms of ischemia, is enough to make the diagnosis of acute myocardial infarction. 

The ideal cardiac enzyme or biomarker needs to be highly specific, highly sensitive, and easily detectable as early as possible in the disease process. Several biomarkers have been developed in the past and will be discussed in this article.

"Cardiac enzymes" is a broad term encompassing several intracellular myocyte components that can be found in serum and measured under certain circumstances such as myocardial ischemia, trauma, myocarditis. In the proper clinical setting, elevation in the level of enzymes present in serum is key in the diagnosis of myocardial infarction. While troponin is the most commonly used cardiac enzyme for diagnosis of myocardial infarction, others exist and may be helpful in some situations.

Specimen Requirements and Procedure

Several assays are available for troponin, and the specific testing information is proprietary information that varies by the assay. Values greater than the 99th percentile are considered positive, but this may also vary by assay and institution.

Diagnostic Tests

Troponin

Troponin is a regulatory protein within muscle cells involved with the interaction of actin and myosin contractile proteins. Troponin I and troponin T assays are available. Cardiac troponin I is found only in cardiac tissue while cardiac troponin T is expressed to a very small degree in skeletal muscle. Contemporary or sensitive cardiac troponin assays have been available for years. Highly sensitive troponin assays are newer and were first approved for clinical use in 2017. With highly sensitive assays, there is a detectable range of troponin that is considered normal, while this is not the case with older sensitive troponin assays where any elevation is often considered significant. Troponin assays are immunoassays and can give false positives with antibody cross-reactivity, although this is rare.  Several troponin assays are available, and levels cannot be compared across assays. Older assays could detect troponin elevations within 3 to 4 hours of myocardial injury and peak at 24 hours. Newer highly sensitive assays detect troponin elevation sooner and vary by assay. Many recommendations based on older assays recommend repeat troponin measurement at 6 to 12 hours, but several strategies now exist with repeat measurement as soon as 2 hours.

In most clinical settings, cardiac troponin is the cardiac enzyme of choice, and other enzymes should not be routinely used. There are many reasons for this, but ultimately, troponin has been shown to be more specific and more sensitive to cardiac injury. Nearly all false-positive troponins are limited to situations where there is antibody cross-reactivity within the testing assay, as troponin is not released from damaged skeletal muscle. CK-MB is released from skeletal muscle, and this can lead to falsely positive elevation. Per gram of myocardial tissue, more troponin is present than CK-MB.

Creatine Kinase/CK-MB

Creatine kinase is a cytosolic protein involved with mitochondrial phosphate transport.   CK exists in three different dimer configurations (MM, MB, BB) of two CK isoenzymes, M and B. Prior to the ubiquitous use of troponin, CK-MB was the mainstay cardiac enzyme for the diagnosis of myocardial infarction. 

Creatine kinase is found in all muscle tissues and is nonspecific for myocyte injury; however, CK-MB is relatively specific for myocardial tissue. CK-MB can be found in serum within 4 to 6 hours of the onset of myocardial ischemia; however, it can take up to 12 hours in some patients. CK-MB levels return to baseline within 36 to 48 hours and, therefore, are sometimes still used to assess for reinfarction after the intervention. Elevations in CK-MB must be interpreted with caution in situations where skeletal muscle injury or disease is also suspected, as CK-MB is released from damaged skeletal muscle. Some institutions will report a ratio of CK/MB to CK to ascertain whether elevations in CK-MB are increased to an extent greater than what would be expected with skeletal muscle injury alone; however, these ratios or indexes have not been demonstrated to improve sensitivity or specificity regarding the diagnosis of myocardial ischemia. CK-MB levels alone are most helpful in situations where myocardial ischemia is suspected and skeletal muscle injury or disease is not suspected. As discussed below, however, troponin is preferred in almost all situations where it is available for use. 

Myoglobin

For many years, CK-MB was the cardiac enzyme of choice for the diagnosis of myocardial ischemia. One problem with this strategy was the length of time from injury to the elevation of CK-MB. Myoglobin was once used in conjunction with CK-MB in an attempt to speed the diagnosis of myocardial injury.  Myoglobin is a very small heme protein found in many tissues. It is rapidly released and has a short half-life. This was of some benefit when CK-MB was the primary assay available; however, as troponin assays have become more sensitive, they have replaced myoglobin for early detection of myocardial injury. High sensitivity cardiac troponin is released earlier from damaged myocardial tissue and to be detectable in serum earlier than myoglobin. 

Heart-Type Fatty Acid Binding Protein

While not available in the United States, heart-type fatty acid-binding protein has been shown in one study to be more sensitive than troponin and myoglobin for early detection of myocardial injury. Troponin was more specific; however, heart-type fatty acid-binding protein has not been studied against high sensitivity troponin and has not been widely adopted for clinical use.  

Lactate Dehydrogenase

Previously used in conjunction with CK-MB, lactate dehydrogenase is also no longer regularly used for diagnosis of myocardial injury. Lactate dehydrogenase is found in many tissues and is therefore not specific. It also takes several hours after onset of injury for levels to become elevated. 

Copeptin

Copeptin is the C-terminal end of the arginine vasopressor precursor protein that is released from the pituitary gland during myocardial ischemia.  Early rule-out strategies using copeptin measurement with standard cardiac troponin assays have not clearly shown an advantage over troponin alone. 

Testing Procedures

Troponin

Cardiac troponin (cTn) assays, which are widely available, differ substantially from each other. Almost all of these are enzyme-linked immunosorbent assays in which there is a capture antibody that captures the material as well as a tag antibody that labels it. Most assays have monoclonal antibodies as capture antibodies, that are specific for the cTn being measured, either cardiac troponin I (cTnI) or cardiac troponin T (cTnT). In order to increase the amount of protein captured and labeled, more than two antibodies are used very often. Each assay is different depending on the antibodies that are being used. Moreover, detection methods and calibration also vary. Therefore, one cannot substitute the value of the test from one assay for another.[6]

Creatine Kinase/CK-MB

Separation of CK into isoenzymes can be accomplished by techniques like electrophoresis, column chromatography, or radioimmunoassay. Clinical laboratories often utilize electrophoresis on an agarose gel or cellulose acetate in combination with band quantification by elution of the electrophoretic bands, or by either fluorometric or spectrophotometric techniques. Electrophoretically, the fraction CK–BB is the most mobile, CK–MB is intermediate, and CK–MM is typically neutral. This helps in the identification of the cardiac-specific CK MB.[7]

Myoglobin

While testing for myoglobin, it is important to distinguish it from hemoglobin as well as to measure accurately. The most widely used definitive tests for the differentiation and quantification of myoglobin from hemoglobin in biological fluids include simple immunochemical methods, which include immunodiffusion, hemagglutination inhibition, and immunoelectrophoresis, which are dependent upon the fact that specific antisera will react only with its homologous antigen.[8]

Interfering Factors

While renal disease can lead to chronically elevated troponin values, the most common cause of a true false positive is immune cross-reactivity with the assay. In these cases, patients will have extremely high values reported that remain elevated. A different assay may be successfully used in some cases.

Results, Reporting, and Critical Findings

The results, as mentioned above, vary according to the type of immunoassays as well as quantification methods.

Clinical Significance

Troponin elevation should always be assumed to be the result of myocardial damage, and further workup and treatment are warranted based on the overall presentation.[9][10][11][12][13] Tests other than troponins are not currently being used, although they had been used in the past.

Enhancing Healthcare Team Outcomes

Myocardial ischemia is a medical emergency that requires to be diagnosed on an urgent basis so that the treatment protocols can begin without any waste of time. Moreover, with atypical presentations, serum markers could be the only relevant method to identify the problem. Therefore, an accurate and sensitive cardiac serum marker like cardiac troponin can help physicians take steps like a percutaneous intervention that can prevent further damage to the myocardium and save the life of a patient.