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. 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. The myocardium has 15% CK-MB isoenzyme and 85% CK-MM. Skeletal muscles contain about 1% to 3% of CK-MB.
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. Hypothyroidism retards the clearance of CK.
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
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.
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.
Serum collected in serum gel tube is required for the measurement of 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 the preferred quantity.
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. 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 which are naturally fluorescent can comigrate with CK-BB and CK-MB during electrophoresis, some of these compounds include bilirubin, aspirin, antidepressants, and benzodiazepines when in high concentrations.
CK-MB elevation was used as one of the criteria for diagnosing acute MI, as its use was increasing 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 since, 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. 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.
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.
(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:
(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.
A CK-MB relative index < 3% is consistent with the skeletal muscle source, whereas the 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.
Hence in patients with clear evidence of lack of trauma, chronic skeletal muscle abnormalities and with a high index of suspicion for AMI, 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, amongst 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.
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 for us to develop assays to measure its level in plasma.
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. 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. 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 peak by about 18 to 24 hours and remains in the detectable levels for 72 to 96 hours.
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 along with the presence of evidence proving the failure of CK-MB relative index along with several other non-AMI cause of CK-MB elevation troponin is has been proven as the biomarker of choice for the detection of myocardial damage of any etiology.
Use of CK-MB despite Troponin being the Biomarker of Choice
Troponin remains in circulation for a longer duration when compared to CK-MB. 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.
Given the significant number of studies and guidelines from 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.
|||van der Veen KJ,Willebrands AF, Isoenzymes of creatine phosphokinase in tissue extracts and in normal and pathological sera. Clinica chimica acta; international journal of clinical chemistry. 1966 Mar; [PubMed PMID: 5943823]|
|||Dawson DM,Eppenberger HM,Kaplan NO, Creatine kinase: evidence for a dimeric structure. Biochemical and biophysical research communications. 1965 Nov 22; [PubMed PMID: 5865501]|
|||Lee TH,Goldman L, Serum enzyme assays in the diagnosis of acute myocardial infarction. Recommendations based on a quantitative analysis. Annals of internal medicine. 1986 Aug; [PubMed PMID: 3524337]|
|||Tsung JS,Tsung SS, Creatine kinase isoenzymes in extracts of various human skeletal muscles. Clinical chemistry. 1986 Aug; [PubMed PMID: 3731455]|
|||Clark GL,Robison AK,Gnepp DR,Roberts R,Sobel BE, Effects of lymphatic transport of enzyme on plasma creatine kinase time-activity curves after myocardial infarction in dogs. Circulation research. 1978 Aug; [PubMed PMID: 27320]|
|||Sobel BE,Markham J,Karlsberg RP,Roberts R, The nature of disappearance of creatine kinase from the circulation and its influence on enzymatic estimation of infarct size. Circulation research. 1977 Dec; [PubMed PMID: 923034]|
|||Karlsberg RP,Roberts R, Effect of altered thyroid function on plasma creatine kinase clearance in the dog. The American journal of physiology. 1978 Dec; [PubMed PMID: 736119]|
|||Trask RV,Billadello JJ, Tissue-specific distribution and developmental regulation of M and B creatine kinase mRNAs. Biochimica et biophysica acta. 1990 Jun 21; [PubMed PMID: 2364108]|
|||Fontanet HL,Trask RV,Haas RC,Strauss AW,Abendschein DR,Billadello JJ, Regulation of expression of M, B, and mitochondrial creatine kinase mRNAs in the left ventricle after pressure overload in rats. Circulation research. 1991 Apr; [PubMed PMID: 2009604]|
|||Keshgegian AA,Feinberg NV, Serum creatine kinase MB isoenzyme in chronic muscle disease. Clinical chemistry. 1984 Apr; [PubMed PMID: 6705202]|
|||Jockers-Wretou E,Grabert K,Müller E,Pfleiderer G, Serum creatine kinase isoenzyme pattern in nervous system atrophies and neuromuscular disorders. Clinica chimica acta; international journal of clinical chemistry. 1976 Nov 15; [PubMed PMID: 1000828]|
|||Jaffe AS,Ritter C,Meltzer V,Harter H,Roberts R, Unmasking artifactual increases in creatine kinase isoenzymes in patients with renal failure. The Journal of laboratory and clinical medicine. 1984 Aug; [PubMed PMID: 6747438]|
|||Schuyler GT,Yarbrough LR, Changes in myosin and creatine kinase mRNA levels with cardiac hypertrophy and hypothyroidism. Basic research in cardiology. 1990 Sep-Oct; [PubMed PMID: 1703406]|
|||Smith AF,Radford D,Wong CP,Oliver MF, Creatine kinase MB isoenzyme studies in diagnosis of myocardial infarction. British heart journal. 1976 Mar; [PubMed PMID: 1259837]|
|||Mair J,Artner-Dworzak E,Dienstl A,Lechleitner P,Morass B,Smidt J,Wagner I,Wettach C,Puschendorf B, Early detection of acute myocardial infarction by measurement of mass concentration of creatine kinase-MB. The American journal of cardiology. 1991 Dec 15; [PubMed PMID: 1746453]|
|||Eisenberg PR,Shaw D,Schaab C,Jaffe AS, Concordance of creatine kinase-MB activity and mass. Clinical chemistry. 1989 Mar; [PubMed PMID: 2646033]|
|||Alpert JS,Thygesen K,Antman E,Bassand JP, Myocardial infarction redefined--a consensus document of The Joint European Society of Cardiology/American College of Cardiology Committee for the redefinition of myocardial infarction. Journal of the American College of Cardiology. 2000 Sep; [PubMed PMID: 10987628]|
|||el Allaf M,Chapelle JP,el Allaf D,Adam A,Faymonville ME,Laurent P,Heusghem C, Differentiating muscle damage from myocardial injury by means of the serum creatine kinase (CK) isoenzyme MB mass measurement/total CK activity ratio. Clinical chemistry. 1986 Feb; [PubMed PMID: 3510780]|
|||Wolfson D,Lindberg E,Su L,Farber SJ,Dubin SB, Three rapid immunoassays for the determination of creatine kinase MB: an analytical, clinical, and interpretive evaluation. American heart journal. 1991 Oct; [PubMed PMID: 1927882]|
|||Adams JE 3rd,Bodor GS,Dávila-Román VG,Delmez JA,Apple FS,Ladenson JH,Jaffe AS, Cardiac troponin I. A marker with high specificity for cardiac injury. Circulation. 1993 Jul; [PubMed PMID: 8319322]|
|||Babuin L,Jaffe AS, Troponin: the biomarker of choice for the detection of cardiac injury. CMAJ : Canadian Medical Association journal = journal de l'Association medicale canadienne. 2005 Nov 8; [PubMed PMID: 16275971]|
|||Katus HA,Remppis A,Scheffold T,Diederich KW,Kuebler W, Intracellular compartmentation of cardiac troponin T and its release kinetics in patients with reperfused and nonreperfused myocardial infarction. The American journal of cardiology. 1991 Jun 15; [PubMed PMID: 1904190]|
|||Adams JE 3rd,Schechtman KB,Landt Y,Ladenson JH,Jaffe AS, Comparable detection of acute myocardial infarction by creatine kinase MB isoenzyme and cardiac troponin I. Clinical chemistry. 1994 Jul; [PubMed PMID: 8013101]|
|||Katus HA,Remppis A,Neumann FJ,Scheffold T,Diederich KW,Vinar G,Noe A,Matern G,Kuebler W, Diagnostic efficiency of troponin T measurements in acute myocardial infarction. Circulation. 1991 Mar; [PubMed PMID: 1999039]|
|||Remppis A,Scheffold T,Greten J,Haass M,Greten T,Kübler W,Katus HA, Intracellular compartmentation of troponin T: release kinetics after global ischemia and calcium paradox in the isolated perfused rat heart. Journal of molecular and cellular cardiology. 1995 Feb; [PubMed PMID: 7776386]|
|||Ricchiuti V,Voss EM,Ney A,Odland M,Anderson PA,Apple FS, Cardiac troponin T isoforms expressed in renal diseased skeletal muscle will not cause false-positive results by the second generation cardiac troponin T assay by Boehringer Mannheim. Clinical chemistry. 1998 Sep; [PubMed PMID: 9732977]|
|||Anderson PA,Malouf NN,Oakeley AE,Pagani ED,Allen PD, Troponin T isoform expression in humans. A comparison among normal and failing adult heart, fetal heart, and adult and fetal skeletal muscle. Circulation research. 1991 Nov; [PubMed PMID: 1934353]|
|||Müller-Bardorff M,Hallermayer K,Schröder A,Ebert C,Borgya A,Gerhardt W,Remppis A,Zehelein J,Katus HA, Improved troponin T ELISA specific for cardiac troponin T isoform: assay development and analytical and clinical validation. Clinical chemistry. 1997 Mar; [PubMed PMID: 9068589]|