creatine kinase MB (CKMB) in serum
Indications
- assessment/diagnosis of myocardial infarction in emergency department or hospital setting
- singleCK-MB levels should not be used for the diagnosis of acute myocardial infarction
- serial samples should be drawn & tested at sequential intervals in connection with measurement of serum troponin-I
Reference interval
Principle
Creatine kinase (ATP: Creatine N-Phosphotransferase,(E.C.2.7.3.2) is the primary catalytic enzyme in the reversible phosphorylation- dephosphorylation reaction;
CK Phosphocreatine + ADP <------------> Creatine + ATP
Creatine kinase exists as a dimeric molecule containing 2 subunits, with each single peptide chain having a molecular weight of 41,000 daltons. Each subunit is also known to have different immunogenic determinants. The 2 subunits can combine to form three different isoenzymes; the MM, the BB & the MB. The CK-BB isoenzyme is found primarily in brain tissue, CK-MM in skeletal muscle & CK-MB is most abundant in cardiac tissue.
The Stratus CK-MB Fluorometric Enzyme Immunoassay is a rapid & sensitive automated procedure for the quantitative determination ofCK-MB in human serum. The methodology of the Stratus CK-MB immunoassay measures immunological activity, not enzymatic activity. Since the concentration of CK-MB protein is measured, the results are expressed in mass units (ng/mL). The CK-MB assay can be completed at the rate of approximately 45 samples/hour & measures a clinically useful range from < 1 to 125 ng/mL.
The Stratus CK-MB assay procedure is based upon the two-site sandwich immunoassay methodology. The sample is pipetted onto the center of a square piece of glass fiber paper where it reacts with the immobilized monoclonal anti-CK-MB antibody. After a short incubation, a conjugate consisting of enzyme-labeled anti-CK-BB directed against a distinct antigenic site on the CK-MB molecule is pipetted onto the reaction zone. During the second incubation period, the labeled antibody reacts with the antibody bound CK-MB forming an antibody/antigen-labeled antibody sandwich. The unbound labeled antibody is later eluted from the field of view of the Stratus Fluorometric Analyzer by applying a substrate wash solution to the center of the reaction zone & removing the unbound fraction by radial elution. By including substrate for the enzyme within the wash solution, initiation of enzyme activity occurs simultaneously with the wash. The enzymatic rate of the bound fraction, which varies directly with the concentration of CK-MB present in the sample, can then be measured by an optical system that monitors the reaction rate via front surface fluorescence. All data analysis functions are performed by the microprocessor within the analyzer
Clinical significance
Quantitation of CK-MB had been the best known marker for diagnosing myocardial infarction until the advent of serum troponin I. Precise determinations of CK-MB at low levels are useful since some patients are known to sustain small infarctions with relatively low quantities of CK-MB & early detection of an infarction (or reinfarction) is possible with a sensitive CK-MB assay.
CK-MB serum levels are generally interpreted in conjunction with serum creatine kinase in the form of a CKMB index.
CK-MB index = (CK-MB/total CK) x 100.
CK-MB index of > 2.5-3.0 is consistent with myocardial infarction
See creatine kinase for distribution of various forms of CK
Determination of CK-MB is most beneficial when samples have been drawn at appropriate time intervals.
Increases
- myocardial ischemia & myocardial infarction
- causes of increased CK-MB other than myocardial infarction:
- most of these conditions will be clinically apparent
- myopathic disorders, are expected to cause a persistently elevated CK-MB
- CK-MB is the first cardiac specific protein detected in elevated levels following mycardial damage
- an increase in CK-MB concentrations is often evident within 3-6 hours following onset of chest pain
- CK-MB reaches peak concentrations within 12-24 hours
- CK-MB concentrations will generally return to normal within 24-72 hours
- reperfusion after PTCA or thrombolytic therapy is associated with higher peak CK-MB levels than persistent occlusion
Preparation
No special patient preparation is required
Specimen
Serum samples only are used for CKMB analysis on the Stratus II analyzer. Collect the blood & allow to clot. Samples should be centrifuged for 15 minutes at approximately 900 rcf. Serum samples are stable for two (2) days when stored at 2-8 degrees C. Samples should not be repetitively frozen & thawed prior to testing. If shipment is required use dry ice.
More general terms
Additional terms
Component of
References
- ↑ Baxter's STRATUS II Operation Manual. Baxter Diagnostic Division, Miami, Florida.
- ↑ Package Insert, Stratus II CK-MB, Baxter Diagnostics Inc., Deerfield, IL, 1992.
- ↑ Baxter's STRATUS II Training Manual, Baxter Diagnostics Inc, Miami, Florida.
- ↑ Panel of 5 tests Laboratory Test Directory ARUP: http://www.aruplab.com/guides/ug/tests/0020414.jsp
- ↑ 5.0 5.1 Mini Panel of 2 tests: Creatine Kinase, Isoenzyme MB . CK-MB Relative Percent Laboratory Test Directory ARUP: http://www.aruplab.com/guides/ug/tests/0080480.jsp