|Values are valid only on day of printing.|
Creatine kinase (CK) activity is found in the cytoplasm of several human tissues; major sources of CK include skeletal muscle, myocardium, and the brain. Cytoplasmic CK isoenzymes are dimers of the subunits M and B (MM, MB, or BB). Brain tissue contains predominantly CK-BB (CK1). Skeletal muscle contains almost exclusively CK-MM (CK3). The myocardium contains approximately 30% of CK-MB (CK2), which has been called the "heart specific" isoenzyme. CK-MB is increased in acute myocardial infarction.
Mitochondrial CK, located at the outer surface of the inner mitochondrial membrane, has been suggested to catalyze the rate-limiting step of energy transfer from mitochondrial adenosine triphosphate (ATP) with the formation of creatine phosphatase (CP). The CP molecule, which is smaller in size than ATP, diffuses to target organelles in the cytoplasm where its energy is transferred to ATP by cytoplasmic CK. CK activity results in nonaerobic production of ATP in muscle tissues during work.
Macro CK refers to at least 2 forms of CK. Macro CK type I is an antibody bound form of cytoplasmic CK. It migrates between CK-MM and CK-MB. Macro CK type II (mitochondrial CK) migrates slightly cathodic of CK-MM. Detection of macro forms of CK is the primary reason for electrophoresis of CK activity.
Detection of macro forms of creatine kinase
Diagnosing skeletal muscle disease, in conjunction with aldolase
Creatine kinase (CK)-MB appears in serum 4 to 6 hours after the onset of pain in a myocardial infarction, peaks at 18 to 24 hours, and may persist for 72 hours.
CK-MB may also be elevated in cases of carbon monoxide poisoning, pulmonary embolism, hypothyroidism, crush injuries, and muscular dystrophy.
Extreme elevations of CK-MB can be associated with skeletal muscle cell turnover as in polymyositis, and to a lesser degree in rhabdomyolysis, as seen in strenuous exercise, particularly in the conditioned athlete.
CK-BB can be elevated in patients with head injury, in neonates, and in some cancers such as prostate cancer and small cell carcinoma of the lung. It can also be elevated in other malignancies; however, the clinical usefulness of CK-BB as a tumor marker needs further investigation.
The presence of macro CK can explain an elevation of total CK. It does not rise and fall as rapidly as CK-MM and CK-MB in muscle injury.
Macro CK type II (mitochondrial CK) is rarely observed. It is only seen in acutely ill patients with malignancies and other severe illnesses with a high associated mortality, such as liver disease and hypoxic injury.
In some patients, the presence of MB is method dependent.
Creatine kinase (CK)-MB values that exceed 50% of the total CK probably reflect unusual B subunit synthesis since heart muscle rarely exceeds 30% MB.
CREATINE KINASE, TOTAL
6-11 years: 150-499 U/L
12-17 years: 94-499 U/L
> or =18 years: 52-336 U/L
6-7 years: 134-391 U/L
8-14 years: 91-391 U/L
15-17 years: 53-269 U/L
> or =18 years: 38-176 U/L
Reference values have not been established for patients that are less than 6 years of age.
Note: Strenuous exercise or intramuscular injections may cause transient elevation of CK.
CREATINE KINASE ISOENZYMES
Apple FS, Quist HE, Doyle PJ, et al: Plasma 99th percentile reference limits for cardiac troponin and creatine kinase MB mass for use with European Society of Cardiology/American College of Cardiology consensus recommendations. Clin Chem 2003 Aug;49(8):1331-1336