What Is a CPK Lab Test?

What is creatine phosphokinase (CPK)?

Creatine phosphokinase (CPK) is an enzyme, which is predominantly present in the heart, brain, and skeletal muscles. Converting creatine into phosphate is the main task of this enzyme. The cells of the body then quickly consume or burn this phosphate and use it as an energy source.

The levels of CPK notably increases when there is muscle damage or stress. The reason is that the muscle cells tend to burst open during muscle injury and the elements then flow into the blood. Thus, the level of CPK in the blood increases since a large amount of this enzyme is present in the muscle cells. 

A person's CK-MB level is often checked in the hospital when signs of a heart attack are present. However, a muscle inflammation is indicated when the level of CPK is elevated during lupus treatment. It may be due to the activity of the disease or an overlapping condition. CPK levels also tend to increase after strenuous activities. For this reason, CPK levels can be tested after several days of rest.

If a person's CPK level remains elevated without exercise or rest, the doctor may order additional tests to help determine which type of CPK is elevated. Such information will help the doctor determine if the damage is in the heart, brain, or skeletal muscles. The CPK level may also increase due to certain medications, such as statins. Inform your doctor if you are taking certain medications.

How is the CPK test performed?

A CPK test requires a blood sample for testing. Blood is usually withdrawn from a vein through the procedure called venipuncture. A repeat test may be done on admitted patients for 2-3 days. A slight pain may be felt when the needle is inserted to collect blood. Some people describe it as a stinging or pricking sensation with some throbbing afterward. 

Most of the time, there are no special preparations needed for this blood test. However, it is very important to inform your healthcare provider if you are currently taking medications. Aside from statins, the following drugs can increase the level of CPK:

• Alcohol
• Cocaine
• Certain anesthetics
• Amphotericin B
• Dexamethasone
• Fibrates

Side Effects

Most people do not develop serious side effects after the blood test, although rare complications may occur, such as:

• Fainting
• Lightheadedness
• Excessive bleeding
• Infection at the site of blood extraction

Seek immediate medical help if you experience any of the above-mentioned symptoms. 

Why is the test performed?

A CPK test is usually done if a person shows signs and symptoms of a heart attack. The doctor may order this blood test to help diagnose a heart attack, identify the cause of chest pain, and determine the extent of heart or muscle tissue damage. 

This test can also help determine a variety of muscle problems or diseases, such as:

• Muscular dystrophy
• Dermatomyositis
• Polymyositis
• Malignant hyperthermia

The most common cause of elevated CPK levels is muscle tissue injury. CPK tends to leak into the bloodstream when a muscle is damaged. 

Other causes of muscle breakdown include:

• Prolonged seizures
• Too much exercise
• Certain medications

Normal CPK Results

Normal CPK values may slightly vary among laboratories. Your healthcare provider is the one who will interpret the result of your blood test. The normal value of total CPK is 10-120 mcg/L. 

Abnormal CPK Results 

People who have any of the following conditions may have elevated CPK levels:

• Convulsions
• Brain injury
• Stroke
• Delirium tremens
• Dermatomyositis
• Polymyositis
• Heart attack
• Electric shock
• Rhabdomyolysis
• Myocarditis
• Lung infarction
• Myopathy
• Muscular dystrophy

The following conditions may also produce high CPK results:

• Pericarditis after a heart attack
• Hyperthyroidism
• Hypothyroidism

The CPK test is also used in diagnosing and treating Duchenne muscular dystrophy (DMD). Usually, if the CPK level is high, it indicates myocardial infarction and some muscular disorders. After 24 to 36 hours of the onset of chest pain, the peak level of CPK occurs in myocardial infarction. The level can also be more than 10 times the normal level, depending on the extent of damage occurred.

CPK-1

CPK-1 is mainly found in the lungs and brain. Elevated levels of CPK-1 may indicate any of the following conditions:

• Brain cancer
• Stroke
• Seizure
• Pulmonary infarction

CPK-2

CPK-2 is usually found in the heart, and when there are elevated values of CPK-2, it may indicate the following conditions:

• Heart attack
• Myocarditis
• Accident-related heart injury
• Electrical injury

Other causes of elevated CPK-2 in the blood include:

• Open heart surgery
• Heart defibrillation

CPK-2 levels in the blood tend to increase after a heart attack, but also often decrease within 48 hours.

CPK-3

CPK-3 is mainly found in the skeletal muscles. Elevated CPK-3 levels may be caused by:

• Muscle damage from injuries
• Prolonged immobility
• Muscle damage due to substance abuse
• Inflamed muscles

Other conditions that tend to increase CPK-3 levels may include:

• Seizures
• Muscular dystrophy
• Electromyography
• Muscle trauma due to surgery, contact sports, or being burned

Additional Blood Tests

Usually, blood is drawn in patients who arrive at the emergency room with a suspected heart attack. The CPK test is initially done in these patients. More specific blood tests will be ordered if the level of CPK increases after four and six hours. This test is called troponin T or I. 

Troponin T and I are proteins found in the blood. Both types of proteins are released when there is heart muscle damage due to a heart attack. A high level of troponin T and I in the blood indicates more damage to the heart. 

Other additional follow-up tests may include the following:

• Kidney function studies
• Imaging studies
• Urinalysis
• Other blood tests

Considerations

The levels of CPK in the blood are higher in people who perform heavy exercises and have greater muscle mass. Compared to other ethnicities, CPK levels tend to be higher in African Americans. Elevated CPK levels may also be caused by muscle damage and certain medications, such as statins. Lower levels of CPK may also occur during pregnancy.

An abnormal CPK increase is often classified as mild, moderate, or severe. These classifications approximately correspond to less than 10 times of the upper normal limit, which is 2,000 IU/L. In general, individuals with renal damage along with elevated values of CPK is a cause for concern when CPK levels reach 5,000 IU/L, especially when patients have co-existing conditions, such as acidosis, sepsis, and volume depletion.