There are 3 isoenzymes of creatine kinase (CK): CK-MM, CK-MB and CK-BB. CK-MM is normally the predominant form found in serum. CK-MB is normally found as a small percentage of total activity, with increases seen in acute myocardial injury as well as certain other states, such as trauma.


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


CK-MB mass assay


< 5 ng/mL


5 to 10 ng/mL

borderline increase

> 10 ng/mL




CK-MB relative index =

= ((CK-MB mass in ng/mL) * 100) / (total CK activity in U/L)



• 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



Timing After AMI

earliest increase

4 - 8 hours

peak level

12 - 24 hours

return to normal

48 -72 hours


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



• 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


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