Creatine Kinase: Definition, Function, Testing

A creatine kinase test is a blood test that measures levels of an enzyme called creatine kinase. This enzyme supports energy production in the body and is particularly important for muscle and brain health.

A creatine kinase test is used to diagnose and monitor several health conditions, such as those that damage the heart, brain, and skeletal muscle.

Creatine phosphokinase (CPK), also known as creatine kinase (CK), is a type of enzyme. Enzymes are proteins that catalyze (speed up) chemical reactions in your body.

CK plays an important role in energy production as it’s responsible for the generation of adenosine triphosphate (ATP). ATP is an energy source for tissues and cells around the body, especially skeletal muscle and the brain.

CK is concentrated in cells that make up your brain and skeletal and heart muscles. There are three types of CK enzymes:

• CPK-MB (cardiac muscle): CPK-MB is mostly found in your heart muscle. 
• CPK-MM (skeletal muscle): CPK-MM is mostly found in your skeletal muscle.
• CK-BB (brain): CK-BB is mostly in your brain tissue.

There is normally only a small amount of CK in the bloodstream due to the normal breakdown of muscle tissues caused by everyday wear and tear, such as exercising and walking.

When significant damage occurs to areas where CK is stored, such as the muscles, heart, and brain, blood levels of CK become elevated. For example, when a person experiences a heart attack, CK levels spike due to damage and death of the cells of the heart. This is why CK tests can help diagnose muscle damage and other serious health conditions. 

A CK test is a blood test. During this test, a healthcare provider uses a needle to take a blood sample from a vein in your arm. The blood sample is then sent to a laboratory to be analyzed and assessed for CK levels. 

The amount of CK in the blood is reported in units (U) of enzyme activity per liter (L) of serum. Normal CK levels vary depending on factors such as sex, body composition, race, and physical activity levels.

Black people have higher CK activity than white people due to differences in the production or clearance of CK. Males tend to have higher CK levels than females due to greater muscle mass.

The normal range for CK in white adults is 46-171 U/L for males and 34-145 U/L for females when using a testing procedure from the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC). This range varies depending on the testing methods and laboratories used.

While high levels of CK aren’t always a cause for concern, elevated levels can indicate several injuries and health conditions, such as muscle damage and muscle-related diseases. 

A CK test is used to diagnose injuries and health conditions, such as:

• Muscle injuries: CK is released when skeletal muscle is injured. Trauma, serious burns, and extreme exercise can all lead to muscle damage.
• Muscle diseases: When muscle tissue is broken down, it releases compounds like electrolytes and proteins, including CK, into the blood. A CK test can help diagnose muscle diseases such as muscular dystrophy, rhabdomyolysis, and myositis.
• Myositis: Myositis is the name for a group of rare conditions that cause muscle weakness due to inflammation. Myositis is an autoimmune disease, meaning the immune system attacks the muscle tissue, causing CK to leak into the bloodstream. 
• Muscular dystrophy (MS): Muscular dystrophy is a group of diseases that cause progressive muscle weakness and muscle loss. CK levels are significantly elevated in all types of muscular dystrophy.
• Rhabdomyolysis: Rhabdomyolysis is a medical condition characterized by rapid muscle tissue breakdown, which causes a spike in CK in the bloodstream. A CK level of more than 1,000 IU/L indicates rhabdomyolysis, with levels over 5,000 IU/L indicating severe rhabdomyolysis. Trauma, extreme exercise, crush injuries, infections, muscle diseases, and certain medications can trigger rhabdomyolysis.

In addition to the medical conditions listed above, CK is sometimes used to diagnose heart attacks. 

A troponin test, which measures blood levels of troponin proteins, is generally considered a better marker for diagnosing a heart attack. However, healthcare providers may also use CK to detect a heart attack.

Troponin is more specific and sensitive than CK, as cardiac troponin only increases when there’s damage to the heart muscle. In contrast, increased CK levels can indicate muscle damage in other areas.

In some cases, CK tests may be used to measure the severity of a stroke and to predict the chance of stroke recurrence.

Blood levels of CK depend on many factors, such as age, gender, race, muscle mass, and physical activity. For this reason, it’s essential to interpret CK test results with the help of a healthcare provider.

A CK test is usually used alongside other tests, including other muscle enzyme tests, to diagnose a medical condition. If your CK levels come back as elevated, you might have some sort of muscle damage. Your healthcare provider may order more bloodwork and additional testing to help them determine whether or not you have an injury or a medical condition.

To narrow down the type of muscle damage, a healthcare provider may order a CK isoenzymes test, which measures the different types of CK in the blood. High levels of CK-MM enzymes indicate damage to your skeletal muscles, while high levels of CK-MB enzymes can be a sign of damage to your heart muscle.

Remember that CK levels can fluctuate, so your healthcare provider may recommend testing them multiple times to assess for muscle damage. 

A CK test is typically ordered by a healthcare provider in an emergency care setting, such as a hospital or urgent care center. A provider may also order this test during a medical examination if a person has experienced trauma, has a certain medical condition, or is experiencing symptoms that could indicate muscle damage.

CK levels are often checked as part of trauma protocols in intensive care units (ICUs) when people have fractures or injured blood vessels caused by blunt force or penetration.

In a hospital setting, your healthcare team may run a CK test if you’ve had any of the following:

• A crush injury or serious trauma
• Significant burns
• A traumatic brain injury or stroke

People who are taking certain medications (e.g., statin medications) or have medical conditions (e.g., muscle disorders like muscular dystrophy) may also need to have their CK levels monitored regularly.

Symptoms

Although not always indicative of muscle damage, the following symptoms can be a sign that a CK test may be needed:

• Muscle pain
• Muscle cramps
• Muscle weakness
• Coordination or balance issues
• Numbness or tingling in your extremities (hands and feet)
• Dark-colored pee
• Swollen legs or feet

Some of these symptoms (e.g., normal muscle pain caused by exercising) aren’t always a cause for concern. However, if you’re experiencing extreme or prolonged muscle pain or any other symptom listed above, it’s best to make an appointment with your healthcare provider. If necessary, they’ll recommend appropriate testing, potentially including a CK test.

Talk to your healthcare provider if you have any questions about CK or CK tests. They can provide more information about CK testing and determine whether it’s appropriate based on your medical history and symptoms.

Creatine kinase (CK) is an enzyme found in your muscles, heart, and brain. It helps support energy production in these areas.

A CK test is a blood test that measures CK levels. It is used to diagnose and monitor several health conditions, such as trauma-related muscle injuries and muscular dystrophy.

Your provider may order a CK test if you have severe or ongoing muscle pain, muscle weakness, balance issues, or tingling in your hands and feet. If you have a muscle disorder, your provider will likely test your CK levels regularly.