Creatine kinase is an enzyme that catalyzes the reversible phosphorylation of creatine by ATP. The end product, phosphocreatine, is a readily available energy source for cells. CK is present in many tissues but skeletal and heart muscles contain the highest concentrations. CK released from skeletal muscle accounts for almost all of the CK activity detected in the plasma of healthy individuals. Circulating CK is cleared by degradation in the liver and reticuloendothelial system and has a circulating half-life of 12 hours.
Historically, CK has been used most often measured to diagnose acute myocardial infarction. Serum CK increases 4 to 6 hours after myocardial necrosis and remains elevated 24 to 48 hours post infarction. Serial measurement of CK activity at 0, 6, and 12 hours after the onset of chest pain yields the greatest sensitivity and specificity. Troponin T has replaced CK as the cardiac marker of choice. However, CK is still a useful indicator of re-infarction within the time period Troponin will be raised from an initial MI (8 days). The ROCHE assay for CK is not affected by haemolysis to the same extent Troponin is and therefore can be a useful alternative where repeatedly haemolysed samples are obtained on a patient.
Plasma CK levels are helpful in diagnosing rhabdomyolysis, because objective clinical signs are often absent. CK is a more sensitive marker of skeletal muscle injury than urine myoglobin. CK levels may exceed 100,000 U/L. CK levels above 6000 U/L are associated with an increased risk of acute tubular necrosis.
Chronic muscle disease, such as the muscular dystrophies and polymyositis- dermatomyositis, elevate plasma CK levels, but not to the same extent as rhabdomyolysis. Muscle trauma and burns also significantly elevate CK. Intramuscular injections and noncardiac surgery elevate total CK. Plasma CK activity is not usually elevated by mild exercise, unless a person is in poor physical conditioning. Severe or prolonged exercise can increase CK levels.
The normal range is very broad skewed toward the top end. Age, sex, and race affect the reference interval. CK levels decline with age due to a reduction in muscle mass. Black men have higher CK levels than non-black men or black women, who have higher levels than white or Asian women. Differences in muscle mass are believed to account for these sex and racial differences. Basal CK levels are higher in the physically fit, reflecting their greater muscle mass.
Some patients may have subnormal CK levels, which may result from reduced muscle mass due to aging, wasting or cachexia. Another explanation is decreased physical activity caused by illness or advanced age. Alcoholics often have low CK levels due to reduced muscle mass. Patients with viral hepatitis may have low levels from reduced physical activity. Some patients with connective tissue disease have low CK activity, but the mechanism is unknown. Patients with thyrotoxicosis and Cushing’s disease frequently have low plasma CK activity, presumably due to altered membrane permeability and diminished enzyme efflux . A similar mechanism may be responsible for the subnormal enzyme activity in patients receiving steroids, estrogens, oral contraceptives, and tamoxifen. Captopril, an antihypertensive drug, produces low plasma CK levels by interfering with disulfide bond formation. Patients with septicemia often have low CK levels due to glutathione depletion, which is necessary for preservation of enzyme activity in the plasma
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