Measurement of CK-MB
(1) electropheresis to measure isoenzymes
(2) mass assay
Increases in CK-MB may be expressed as:
(1) an increase in percent of total CK activity (electropheresis)
(2) an increase in mass assay, as ng/mL
(3) CK-MB relative index
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CK-MB mass assay
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Interpretation
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< 5 ng/mL
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normal
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5 to 10 ng/mL
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borderline increase
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> 10 ng/mL
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increased
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CK-MB relative index =
= ((CK-MB mass in ng/mL) * 100) / (total CK activity in U/L)
where:
• this approximates a percentage except for the issue of incompatible units.
• increased if >= 5
• if total CK elevated, a borderline increase is in range 2-5
• if total CK is not elevated, normal is < 5
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Course
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Timing After AMI
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earliest increase
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4 - 8 hours
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peak level
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12 - 24 hours
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return to normal
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48 -72 hours
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CK-MB elevations in muscle disease:
(1) a small amount of CK-MB is present in skeletal muscle, which can lead to significant serum levels in massive skeletal muscle injury
(2) in trauma, the rise in CK-MB shows a rise then fall, but the CK index is low
(3) in myopathy the elevation in CK-MB tends to be constant
Limitations:
• rarely patients with heterophilic antibodies can have spurious elevations of CK-MB
• patients with both skeletal muscle and cardiac injury (post open heart surgery, chest trauma) can be difficult to diagnose with confidence
Clinical utility:
• sensitivity: 94-100%
• specificity: 93-100%
• ROC curve analysis shows diagnostic accuracy indistinguishable from troponin I for diagnosis of acute myocardial infarction
• serial monitoring more useful than single determinations
