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Clinical Chemistry

Cardiac-related tests

Creatine Kinase isoenzyme MB (CKMB)


Author: Larry Bernstein, M.D. (see Reviewers page)
Revised: 10 March 2011, last major update March 2011
Copyright: (c) 2003-2011, PathologyOutlines.com, Inc.

Definition
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● Sensitive and specific test for myocardial infarction, now widely replaced by troponin

Physiology
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● Catalyzes the conversion of creatine to phosphocreatine, consuming adenosine triphosphate (ATP) and generating adenosine diphosphate (ADP)

Clinical use
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● CK isoenzyme MB rises some 4-6 hours after the onset of chest pain, peaks within 12-24 hours, and returns to baseline levels within 24-48 hours
● CK-MB is usually ordered, along with total CK in persons with chest pain to determine whether the pain is due to myocardial infarction
● May also be ordered in a person with a high CK to determine whether damage is in the heart

Test methodology
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● Electrophoresis: serum creatine kinase (CK) is separated into 3 isoenzymes by electrophoretic separation on agarose gel; colorimetric results allow for improved workflow management as the gels do not have to be scanned immediately; the permanent patterns combined with a clear gel background means scanning and quantitation are easy
● Immunoassays are also commonly used

Test indications
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● Troponin has largely replaced CK-MB in many hospitals, although some centers still rely on CK-MB (Wikipedia)

Test limitations
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● Some patients have a variant of CK-BB called "Macro CK", which complexes to IgG or IgA antibody; it migrates between MM and BB on the gel, and may falsely increase CK-MB values
● CK-MB can be elevated with massive rhabdomyolysis, even though the concentration is low in skeletal muscle
● Electrophoresis of CK with values of total CK under 100 U/L may cause false positive CK-MB values

Reference ranges
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● If the value of CK-MB is elevated and the ratio of CK–MB to total CK (relative index) is more than 2.5–3, it is likely that the heart was damaged
● A high CK with a relative index below 2.5-3.0 suggests that skeletal muscle and not cardiac muscle was damaged

Additional references
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Early studies:: Proc Natl Acad Sci USA 1974;71:1384, Circulation 1975;51:855, Clin Chem 1980;26:861, Lancet 1977;2:319, Clin Chem 1983;29:590

End of Clinical Chemistry > Cardiac-related tests > Creatine Kinase isoenzyme MB (CKMB)


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