Creatine Kinase (CK)

A creatine kinase (CK) test may be used to detect inflammation of muscles (myositis) or muscle damage due to muscle disorders (myopathies) such as muscular dystrophy or to help diagnose rhabdomyolysis if a person has signs and symptoms. CK may be ordered along with other blood chemistry tests such as electrolytes, BUN or creatinine (to evaluate kidney function). A urine myoglobin may also be ordered.

A person may have muscle injury with few or nonspecific symptoms, such as weakness, fever, and nausea, that may also be seen with a variety of other conditions. A healthcare practitioner may use a CK test to help detect muscle damage in these cases, especially if someone is taking a drug such as a statin, using ethanol or cocaine, or has been exposed to a known toxin that has been linked with potential muscle damage. In those who have experienced physical trauma, a CK test may sometimes be used to evaluate and monitor muscle damage.

A series of CK tests may be used to monitor muscle damage to see if it resolves or continues. If a CK is elevated and the location of the muscle damage is unclear, then a healthcare practitioner may order CK isoenzymes or a CK-MB as follow-up tests, to distinguish between the three types (isoenzymes) of CK: CK-MB (found primarily in heart muscle), CK-MM (found primarily in skeletal muscle), and CK-BB (found primarily in the brain; when present in the blood, it is primarily from smooth muscles, including those in intestines, uterus or placenta).

The CK test was once one of the primary tests ordered to help diagnose a heart attack, but in the U.S., this use of CK has been largely replaced by the troponin test. However, the CK test may sometimes be used to help detect a second heart attack that occurs shortly after the first. (For more, see Common Questions #4.)

A CK test may be ordered when muscle damage is suspected and at regular intervals to monitor for continued damage. It may be ordered when a muscle disease (myopathy) such as muscular dystrophy is suspected or when someone has experienced physical trauma, such as crushing injuries or extensive burns. The test may be ordered when a person has symptoms associated with muscle injury such as:

• Muscle pain or aches
• Muscle weakness
• Dark urine (The urine may be dark because of the presence of myoglobin, another substance released by damaged muscles that can be harmful to the kidneys.)

Testing may be ordered when a person has nonspecific symptoms, especially when taking a drug or after an exposure to a substance that has been linked with potential muscle damage.

A high CK, or a rise in levels in subsequent samples, generally indicates that there has been some recent muscle damage but will not indicate its location or cause. Serial test results that peak and then begin to drop indicate that new muscle damage has diminished, while increasing and persistent elevations suggest continued damage.

Increased CK levels may be seen in some muscular disorders (myopathies), which have a wide variety of causes. People may have CK levels that are significantly to greatly increased, depending upon the severity of muscle damage. Those who have rhabdomyolysis may have CK levels that are 100 times normal levels and occasionally even higher.

Increased CK may be seen with, for example:

• Recent crush and compression muscle injuries, trauma, burns, and electrocution
• Inherited myopathies, such as muscular dystrophy
• Hormonal (endocrine) disorders, such as thyroid disorders, Addison disease or Cushing disease
• Strenuous exercise
• Prolonged surgeries
• Seizures
• Infections – viral (such as influenza and HIV), bacterial, fungal, and parasitic (such as malaria)
• Connective tissue disorders (e.g. lupus, rheumatoid arthritis)
• Celiac disease
• Renal failure
• In critically ill patients
• High fever accompanied by shivering
• A blood clot (thrombosis) blocking the flow of blood
• Any drug or toxin that interferes with muscle energy production or increases energy requirements

Normal CK levels may indicate that there has not been muscle damage or that it occurred several days prior to testing.

Moderately increased CK levels may be seen following strenuous exercise such as in weight lifting, contact sports, or long exercise sessions.

People who have greater muscle mass have higher CK levels than those who don't; for this reason, men generally tend to have higher values than women.

Any kind of muscle damage, including shots (injections), can temporarily increase CK.

A low CK level may be seen in early pregnancy.

Rhabdomyolysis is the rapid breakdown of muscle tissue. This condition can be caused by serious physical, chemical, or biological injury to muscles. Examples of causes include:

• Trauma, crushing injuries (e.g., car accidents, disasters such as earthquakes)

• High-voltage electrical shock

• Serious burns

• Blood clot (thrombosis) that blocks blood flow

• Toxins (e.g., heavy metals, snake venom, carbon monoxide)

• Infections (e.g., HIV, influenza, Streptococcus) — more common cause in children than adults

• Inherited genetic and metabolic disorders that affect muscles' ability to get or use energy

• Diseases such as muscular dystrophy and underlying conditions such as uncontrolled diabetes, hypothyroidism, and hyperthyroidism

• Several drugs (examples include drugs of abuse (ethanol, cocaine), some antibiotics, antidepressants, corticosteroids, lithium, salicylates and statins)

Complications can result from the rapid release of cell contents into the blood. This has been known to cause damage to kidneys (acute kidney injury, AKI) and disseminated intravascular coagulation (DIC). Once diagnosed and depending on the extent of injury, a person with rhabdomyolysis may be treated with intravenous fluids and other supportive care as well as procedures used to protect organs (e.g., dialysis to prevent/limit kidney damage).