Creatine Kinase (CK) Test

The small amount of CK that is normally present in the blood comes primarily from skeletal muscles. Conditions that cause muscle damage may lead to an increase in blood CK. For example, inflammation of muscles, called myositis, can increase CK concentrations.

Rhabdomyolysis, a severe breakdown of skeletal muscle tissue that causes muscle pain, tenderness, weakness and swelling, is associated with significantly elevated levels of CK, often 100 times normal. Measurement of CK is used to assess the extent of muscle damage and to monitor its progress.

Measurement of CK is often used for the assessment of muscle related side effects from drugs such as statins and as part of the investigation of patients with suspected muscular dystrophy.

Measurement of CK is requested in people who have sustained severe muscle trauma, particularly from crush injuries, burns or electrocution, and are likely to develop rhabdomyolysis.

CK may also be requested in those who develop symptoms or signs of less severe muscle damage

• after being immobile for a long time on a hard surface, for example during an operation or after a stroke, drugs or alcohol
• following very severe exercise
• after a fit
• during a severe infection
• while taking certain medicines, for example a statin to lower cholesterol

Rhabdomyolysis is likely if the urine develops a reddish-brown colour and gives a positive dip-stick test for blood. (Myoglobin, a red protein released from damaged muscle, reacts with the dip-stick in the same way as haemoglobin from blood cells.) The excretion of myoglobin by the kidney can potentially cause kidney failure.

Blood concentrations of CK (or its CK-MM isoenzyme) are also requested as a first step in the diagnosis of muscular dystrophy in infants or children who

• have difficulty walking
• have difficulty standing
• have difficulty lifting weights
• need help climbing stairs

The test may subsequently be used to screen family members.

A high CK concentration, or a result that goes up from the first to a second or later samples, generally indicates that there has been some recent muscle damage. Serial test results that peak and then begin to drop indicate that new muscle damage has diminished, while increasing and persistent elevations suggest continuing damage.

For the detection of heart muscle damage, myocardial infarction, changes in serum CK and its heart tissue (MB) isoenzyme have been largely replaced by the more heart specific nonenzymatic markers, cardiac troponin I or T.

People who have greater muscle mass have higher normal CK levels. For example, a young fit male will have more muscle mass than an elderly female and hence a higher CK concentration. Normal exercise such as running or cycling and drug injections into muscle can increase CK concentrations.

CK concentrations have been found to be normally higher in black race populations.

Myoglobin released from damaged muscle into plasma is filtered by the kidneys into the urine but can precipitate in the kidney tubules, obstructing urine formation. This is a particular risk if shock lowers blood pressure and reduces the rate of urine production.