Creatine kinase is an widespread enzyme catalyzing the phosphorylation of creatine to creatine phosphate. It occurs as three different isoenzymes, each composed of two polypeptide chains, B and M:
- skeletal muscle has a very high CK content; usually, 98% CK-MM and 2% CK-MB
- cardiac muscle also has a very high CK content; usually, 70-80% CK-MM and 20-30% CK-MB
- brain, prostate, thyroid, gut and lung has predominantly CK-BB
- plasma has predominantly CK-MM with less than 6% CK-MB
Usually, the heart is the only tissue in which the amount of CK-MB exceeds 5%. Exceptions are patients with muscle disease and in athletes, in which the skeletal muscle content of CK-MB may rise to 5-15%.
Normally the laboratory provides total CK levels. Normal ranges are 24-170 Units/litre in women, 24-195 in men. A heparinised plasma is required.
The clinician must be aware of the the rare false positives provided by macro CK-1 and macro CK-2.