Creatine Phosphokinase (CPK) Dx

Synonyms: Creatine kinase, CK, CPK.

CPT Code: 82550

Description: CPK is an intracellular enzyme found in high concentrations in skeletal muscle, myocardium, and brain. Damage to these tissues results in elevated serum levels of CK. Three isoforms are used to determine the tissue origin of serum CK: skeletal muscle (MM), myocardium (MB), and brain (BB).

Method: Analysis may be part of a selected automated chemistry profile or may be ordered separately. Serum should be collected in a plain red-top tube. Avoid hemolysis. All determinations of CK levels should be done before invasive diagnostic procedures such as electromyography or muscle biopsy. Ultraviolet spectrophotometry is most common.

Normal Values: Values depend on the method used but generally range from 50 to 200 U/L for males; values for females are 25% lower. Black individuals (males more so than females) may have CK levels above normal values. Such values do not correlate with muscle mass and are not associated with an occult myopathic process. Normal values are seen in renal or pulmonary infarction, pericarditis, thyrotoxicosis, and steroid myopathy.

Increased in: Most patients with dermatomyositis or polymyositis have elevated serum levels of CK. Unless the clinical picture is unclear (e.g., in a patient with chest pain as well as limb weakness), determination of the isoenzyme pattern is not necessary. Although skeletal muscle (MM) is the usual isoenzyme pattern in patients with myositis, elevated MB fractions may occur because of inflammatory damage to, or regeneration of, skeletal muscle, which can express the MB isoform. If CK levels are elevated at the outset of myositis, serial measurements may provide a useful index of therapeutic response.

A significant minority (35%) of patients with active dermatomyositis or polymyositis do not show CK elevations, and in these cases, measurement of other muscle enzymes such as aldolase may be useful. Low levels of CK, sometimes below the normal range, may predict a poor prognosis, especially in patients with malignancy-associated dermatomyositis. Reasons for the lack of CK elevation in some patients with polymyositis/dermatomyositis are not clear but may include the loss of muscle mass or the presence of circulating inhibitors of this enzyme.

Other causes of CK elevation include alcoholic myopathy, myxedema (hypothyroidism), malignant hyperthermia syndrome, Duchenne muscular dystrophy, seizure, eosinophil-myalgia syndrome, late pregnancy (parturition), moderate to severe hemolysis, cocaine use, rhabdomyolysis, cerebrovascular accident, myocardial injury, cardioversion, or muscle trauma. Intramuscular injections and vigorous exercise can also cause CK elevations and should be avoided before phlebotomy.

Decreased in: Those with low muscle mass, severe dermatomyositis or polymyositis, alcoholic liver disease, early pregnancy (20 weeks), and RA may show low CK values.

Indication: CK may be useful in the diagnosis and treatment of inflammatory myositis, muscular dystrophy, myocardial disease, and rhabdomyolysis.

Cost: $30–100.

BIBLIOGRAPHY

Wei N, Pavlidis N, Tsokos G, et al. Clinical significance of low creatine phosphokinase values in patients with connective tissue diseases. JAMA 1981;246:1921–1923.PMID: 7288967

Worrall JG, Phongsathorn V, Hoope RJL, et al. Racial variation in serum creatine kinase unrelated to lean body mass. Br J Rheumatol 1990;29:371–373.PMID: 2224407