CK (creatine kinase) is an enzyme (protein) found in several tissues, including muscles and the heart. Depending on the tissue, different forms of CK are present: CK-MM, primarily present in skeletal muscles, CK-MB with the highest percentage in the heart, and CK-BB, which comes from the brain and smooth muscle, such as the intestinal walls. The specific CK-MB assay is used primarily to diagnose myocardial infarction when total CK levels are high and a troponin assay is not available. CK-MB appears in the blood 3 to 6 hours after the onset of the infarction. It reaches a maximum level in 12 to 24 hours, before returning to normal in 48 to 72 hours. If a second infarction occurs in the interim, the elevated CK-MB level will be prolonged.
A high CK-MB usually reflects a heart muscle injury (infarction, inflammation, trauma, surgery or heart biopsy). Intense muscle exercise can sometimes cause abnormal CK-MB because skeletal muscles contain approximately 7% CK MB. More rarely, kidney failure, chronic muscle disease or hypothyroidism can also increase CK-MB.