What Are CPK Muscle Enzymes?

Creatine phosphokinase, or CPK, is an enzyme that is present in the heart, brain and skeletal muscles. CPK is more commonly known as creatine kinase, or CK. CK is released from the cells of these tissues when they are injured or do not receive enough oxygen. Therefore, your doctor probably won't order it unless he or she thinks there is damage to one of these tissues.

CPK is most commonly used to diagnose a heart attack (Image: Comstock/Stockbyte/Getty Images)

Types

According to Lab Tests Online, there are three types of CK: CK-MM, CK-MB and CK-BB. CK-MM is present mostly in the heart and skeletal muscles, CK-MB is present mostly in the heart, and CK-BB is present mostly in the brain.

Indications for Testing

CK levels are most commonly used to diagnose a heart attack in patients who are having chest pain, notes Lab Tests Online. It is also used to diagnose injuries to the skeletal muscles in those with weakness or muscle pain, as well as injuries to the brain.

Normal Levels

Normal levels of total CK are 55 to 170 international units per liter, or IU/L, in adult males and 30 to 135 IU/L in adult females, reports "Mosby's Manual of Diagnostic and Laboratory Tests." The normal level in newborns is 68 to 580 IU/L. "Clinical Methods" states that CK-MB makes up about three to five percent of total CK, or about five to 25 IU/L.

Causes of Elevated Levels

Elevated levels of CK can occur when there is damage to your heart, brain or skeletal muscle cells. According to "Mosby's Manual of Diagnostic and Laboratory Tests," elevated levels of CK-MB may be caused by heart attack, myocarditis, ventricular arrhythmias, malignant hyperthermia and shock. Conditions that may raise CK-BB include stroke, brain injury, cancer of the brain, subarachnoid hemorrhage, seizures, shock and pulmonary infarction. CK-MM levels may be raised due to myositis, rhabdomyolysis, muscular dystrophy, recent surgery, trauma to the muscles, convulsions, delirium tremens, electroconvulsive therapy, low levels of potassium in the blood and low levels of thyroid hormone.

Considerations

There are several things that should be taken into consideration when measuring CK levels. According to "Mosby's Manual of Diagnostic and Laboratory Tests," intramuscular injections, strenuous exercise and surgery can all elevate CK levels. Several medications including ampicillin, amphotericin B, aspirin, anticoagulants, dexamethasone, furosemide, clofibrate, captopril, lovastatin, lidocaine, succinylcholine, lithium, propranolol and morphine may also elevate CK. Low levels of CK may be present in the beginning of pregnancy. One should also take into account the patient's muscle mass, as it is directly correlated to the level of CK. "Clinical Methods" also notes that CK is a "sensitive but nonspecific test" for diagnosing heart attacks because CK is present in tissues other than the heart, such as the skeletal muscle.

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