Introduction
CK-MB is an isoenzyme of creatine kinase. Creatine kinase dephosphorylates creatine phosphate to creatine, providing the energy required for ATP regeneration.
In 1966 creatine kinase isoenzymes were identified in various tissues.[1] The isoenzymes of CK are dimers of either type B or type M polypeptide chains, BB isoenzymes are found in the central nervous system, MM isoenzyme is a principal component in adult skeletal muscles.[2] The myocardium has 15% CK-MB isoenzyme and 85% CK-MM.[3] Skeletal muscles contain about 1% to 3% of CK-MB.[4]
Pathophysiology
Clearance of CK from Blood or Plasma
Creatine kinase (CK) and its isoenzymes are inactivated in the lymph by proteolysis. Abnormal liver function or renal function does not affect the clearance of CK in a significant manner, and creatine kinase is not excreted in the urine.[5][6] Hypothyroidism retards the clearance of CK.[7]
CK-MB can also be elevated in circulation in the absence of acute myocardial infarction (AMI), and this is due to increased amounts of B subunit production in injured skeletal muscle as it does during fetal development, ontogeny recapitulates phylogeny[8][9]
Rhabdomyolysis, intense exercise, trauma result in transient elevation of CK and CK-MB; CK-MB is present in skeletal muscles as well, albeit in lesser concentrations. Chronic skeletal muscle disorders such as autoimmune myopathies and inflammatory myopathy can result in persistently high CK-MB levels in the plasma due to ongoing injury and repair.[10][11][12][13] Damage to the myocardium releases CK-MB, and since the myocardium contains the largest percentage of CK-MB, patients with rapidly rising and falling CK-MB exceeding the reference range of normal should be considered as having AMI until proven otherwise.
Specimen Requirements and Procedure
Serum collected in serum gel tube is required to measure creatine kinase levels; when serum gel tube is not available, a red top tube is acceptable. However, red top tubes must be centrifuged and aliquoted within 2 hours of collection; serum gel tubes must be centrifuged within 2 hours.
The minimum amount of serum required to perform the test is 0.25 ml, but 1 ml is preferred.
Interfering Factors
Measurement of CK-MB, Interfering Factors, and Negating Techniques
CK-MB was initially separated using gel electrophoresis, and densitometry was used to quantify the activity of CK-MB isoenzyme in blood.[14] Adenylate kinase released by red blood cells results in false elevation of CK activity. Currently, laboratories add reagents to inhibit adenylate kinase activity. Multiple commonly occurring compounds that naturally fluoresce can comigrate with CK-BB and CK-MB during electrophoresis; some of these compounds include bilirubin, aspirin, antidepressants, and benzodiazepines when in high concentrations.[12]
CK-MB elevation was used as one of the criteria for diagnosing acute MI; as its use increased in frequency in the late 1980s and 1990s, it became evident that despite its high sensitivity in detecting Acute MI, the specificity of CK-MB activity was low. Better methods to measure CK-MB have been developed to improve the specificity of CK-MB. CK-MB mass measurements using Immunoenzymometryic assays containing monoclonal antibodies binding to M and B subunits individually were proven to be highly specific and more sensitive than CK-MB activity measurement.[15] Even assays using monoclonal antibodies have been found to have elevations in CK-MB mass due to the cross-reactivity of alkaline phosphatase in plasma with stabilizing agents found in commercial reagents.[16]
Clinical Significance
Diagnosis of Acute Myocardial Infarction
According to the Universal consensus statement from the American College of Cardiology and the European Society of Cardiology, acute MI is defined by the presence of at least one of the below criteria.[17]
(1) Typical rise and slow fall of troponin T or rapid rise and fall (CK-MB) of biochemical markers of myocardial necrosis with at least one of the following:
- Ischemic symptoms
- Development of pathologic Q waves on the ECG
- ECG changes indicative of ischemia (ST-segment elevation or depression)
(2) Pathologic findings of an acute MI
CK-MB concentration gradually rises in blood in 4 to 6 hours after onset of chest pain, peaks by around 24 hours, and returns rapidly to baseline in 48 hours.
However, by the time of the joint statement from ACC and ESC in 2000, troponin T testing was proven to be more specific to the myocardium, and it will be discussed briefly later in the article.
Non-acute MI Causes of CK-MB Elevation
As discussed above, the skeletal muscle and myocardial cell death of any etiology will cause an elevation of CK-MB. Listed below are multiple other causes of CK-MB elevation in plasma.
False elevations in CK-MB occur in the presence of atypical CK isoforms, macrokinases, and adenylate kinase; however, these false elevations can be eliminated by adding reagents to testing kits.
Cardiac etiology - myocarditis, cardiac surgery can damage heart muscle resulting in elevation of CK-MB.
Peripheral sources - rhabdomyolysis, myositis, inflammatory myopathies, trauma, medications (daptomycin, statins, antiretrovirals)
To differentiate the elevation of CK-MB for cardiac etiology versus skeletal muscle source, we can calculate the CK-MB relative index (CK-MB RI) by using the below formula.
- CK-MB RI = CK-MB (ng/mL) /CK (ng/mL) X 100
A CK-MB relative index < 3% is consistent with the skeletal muscle source, whereas a relative index > 5% is consistent with the cardiac source of CK-MB. However, prior studies in patients with trauma and patients with chronic skeletal muscle abnormalities have demonstrated the failure of CK-MB Relative index in differentiating skeletal muscle sources of CK-MB from myocardial cell death.[18][19][20]
Hence in patients with clear evidence of lack of trauma, chronic skeletal muscle abnormalities, and a high index of suspicion for AMI, the use of CK-MB RI can increase the specificity of CK-MB testing
Miscellaneous causes include hypothyroidism, renal failure, alcohol intoxication, pregnancy, and certain types of malignancies.
Current Biomarker Use
As explained earlier, following the WHO Criteria for diagnosis of AMI, multiple cardiac biomarkers were being used to diagnose acute myocardial infarction, among them, CK-MB was being used as the most sensitive and specific marker for diagnosis of AMI, detection of reperfusion, and estimating the size of myocardial infarction in the 1990s. During this time, troponin was evaluated as potentially a more specific biomarker for myocardial infarction when compared to CK-MB.[17]
Troponin is a protein complex of 3 units, troponin T, troponin I, and troponin C, present in the actin filament of the skeletal and myocardial muscle cells. There are multiple isoforms of troponin T and troponin I, one of which is specific to cardiac muscle, and it is not expressed in adult skeletal muscle allowing the development of assays to measure its level in plasma.[21]
Troponin is present in the myocardium as a 3 units complex in the contractile apparatus attached to the actin filament of the tropomyosin complex, however similar to CK-MB, there is unbound/free troponin in the cytosol of myocardial cells, which is known as the cytosolic pool. In the event of myocardial damage, the unbound troponin is first released[22][23]. This unbound troponin is about 6% of the total troponin in the myocardium. The rest of the troponin, which is bound to the actin, is released slowly with structural damage and results in the prolonged duration of elevated troponins in the plasma.[23][24] Troponin elevation > 99th percentile is used as the cutoff value for the diagnosis of AMI. Troponin concentration begins to rise 4 to 6 hours after onset of symptoms, peaks by about 18 to 24 hours, and remains in the detectable levels for 72 to 96 hours.[25][26]
Troponin is more specific to the cardiac muscle when compared to CKMB, and current assays for troponin are more sensitive and specific than the assays for CK-MB measurement. Given the expression of CK-MB in skeletal muscle and the presence of evidence proving the failure of CK-MB relative index and several other non-AMI causes of CK-MB elevation, troponin has been proven as the biomarker of choice for the detection of myocardial damage of any etiology.[18][19]
Use of CK-MB despite Troponin being the Biomarker of Choice
Troponin remains in circulation for a longer duration when compared to CK-MB.[27][28] In conditions where reinfarction is suspected, CK-MB may be useful to classify a new event due to its shorter duration of elevation at detectable levels in plasma. However, after the advent of troponin and the current aggressive interventional approach to AMI, and due to lack of literature comparing CK-MB against troponin in the diagnosis of reinfarction, the use of CK-MB has declined.
Enhancing Healthcare Team Outcomes
Given the significant number of studies and guidelines from the American College of Cardiology recommending the use of troponin for the diagnosis and ruling out of acute coronary syndromes instead of CK-MB, decreasing the use of CK-MB in hospital and outpatient setting requires an interprofessional team of healthcare professionals that includes a nurse, laboratory technologists, pharmacist and several physicians in different specialties especially cardiologists and cardiothoracic surgeons. Specialty-trained nurses are involved in the ordering and interpretation of this test.