The creatine kinase (CK) isoenzymes panel measures the three major forms of the CK enzyme found in different parts of the body—CK-MM, CK-MB, and CK-BB—to help pinpoint the origin of tissue damage, especially when total CK is elevated. CK is a key enzyme involved in cellular energy production. It helps regenerate adenosine triphosphate (ATP)—the energy currency of the cell—by converting phosphocreatine into creatine and ATP. When muscle cells are damaged, CK leaks into the bloodstream. However, since CK is present in several types of tissue, measuring total CK alone can’t tell us which tissue is affected. That’s where the CK isoenzymes panel becomes valuable.
The panel breaks down total CK into its three isoforms:
- CK-MM: Found mostly in skeletal muscle, accounting for 95–98% of CK in healthy people. Elevated in muscle injuries, rhabdomyolysis, muscular dystrophies, and after intense exercise.
- CK-MB: Found in heart muscle. Historically used to diagnose acute myocardial infarction (heart attack), CK-MB rises within 4–6 hours after cardiac injury and returns to normal within 2–3 days. Troponin has largely replaced it in modern cardiology, but CK-MB is still useful for detecting reinfarction or cardiac injury in settings where troponin may be elevated chronically.
- CK-BB: Found mainly in the brain and smooth muscle. Rarely seen in serum under normal conditions. Its presence may indicate brain injury, certain cancers, or neonatal brain damage. Because CK-BB is unstable in blood and rapidly cleared, elevated levels can be difficult to catch.
The CK isoenzymes panel is useful in the following scenarios:
- When total CK is elevated and the source of injury is unclear
- To help differentiate between cardiac and skeletal muscle injury
- In newborn screening for muscular dystrophy (CK-MM)
- To assess brain injury or stroke (CK-BB, though rarely used clinically)
- To monitor for reinfarction when troponin levels are persistently high
