Enzymes are proteins that help chemical processes in the body. One important enzyme for muscle function is creatine phosphokinase (CPK), which can be measured with a blood test.
CPK can be broken down into three separate parts:
- CPK-1 is mainly found in your brain and lungs.
- CPK-2 is mostly found in your heart.
- CPK-3 is found in your skeletal muscle.
When these parts of your body become damaged due to injury or disease, CPK enzymes can be released into your bloodstream. The CPK isoenzymes test checks the levels of these enzymes in your blood. This can help your doctor identify the areas of your body that have been damaged.
The CPK isoenzymes test is a simple blood test that involves minimal preparation and risk. The blood sample will be sent to a laboratory for analysis, and your doctor will explain the results to you.
The CPK isoenzymes test may still be used in some cases, but over the past decade, most doctors have transitioned away from it. Instead, a troponin test is often performed to evaluate damage to your heart muscle. A troponin test measures the levels of proteins called troponin T and troponin I in the blood. These proteins are released when your heart muscle becomes damaged due to a heart attack or other serious heart condition. The troponin test procedure is similar to that of a CPK isoenzymes test.
A CPK isoenzymes test is usually done in the emergency room if you have the symptoms of a heart attack. Your doctor may order a CPK blood test to:
- help them diagnose a heart attack
- find the cause of your chest pain
- find out how much heart or muscle tissue has been damaged
The test can also determine whether you carry the gene for muscular dystrophy. Muscular dystrophy is a group of diseases that causes muscle loss and weakness over time. A CPK isoenzymes test can detect various muscle diseases or issues, including:
- dermatomyositis, which is an inflammatory disease that affects the skin and muscles
- polymyositis, which is an inflammatory disease that causes muscle weakness
- malignant hyperthermia, which is an inherited disease that causes muscle contractions
- other conditions that may cause muscle breakdown, such as over-exercising, certain medications, or prolonged seizures.
The CPK isoenzymes test is similar to other blood tests. It doesn’t require any fasting or special preparation.
Before you schedule your blood test, it’s important to tell your doctor about any over-the-counter and prescription medications you’re taking. Some substances can cause elevated CPK, including:
- drugs that lower cholesterol
- amphotericin B, which is an antifungal medication
Other factors may cause elevated test results, including:
- vigorous exercise
- recent surgery
- intramuscular injections, such as vaccines
- cardiac catheterization, which is when a catheter is inserted into a vein in your arm, groin, or neck and threaded to your heart
Make sure to tell your doctor if you’ve recently experienced any of these events.
The blood test should only take a few minutes. A healthcare provider will use a topical antiseptic to clean a small area of your arm, usually on the inside of your elbow or on the back of your hand. They’ll tie an elastic band around your upper arm to create pressure and make it easier to find your vein.
Once they find your vein, they’ll insert a sterile needle into it and draw your blood into a small vial. You may feel a slight prick as the needle goes in, but the test itself isn’t painful. After the vial is filled, the needle and elastic band will be removed. A bandage will then be placed over the puncture site.
The vial will be labeled and sent to a laboratory. The test results will be sent to your doctor, who will explain them to you.
In some cases, your doctor may want to repeat the test over several days to see if your enzyme levels change. Finding varying levels can help with the diagnosis.
Your arm may feel sore where the needle was inserted. You may also have some mild, temporary bruising or throbbing near the puncture site. You’ll likely feel more discomfort if the healthcare provider had difficulty accessing a vein and multiple puncture wounds were made.
Most people don’t have any serious or lasting side effects. Rare complications of a blood test include:
Call your doctor immediately if you experience any of these symptoms.
CPK-1 is found primarily in your brain and lungs. Elevated CPK-1 levels could indicate:
- a brain injury due to stroke or bleeding in the brain
- a seizure
- brain cancer
- a pulmonary infarction, or the death of lung tissue
CPK-2 is found mostly in your heart. Elevated levels of CPK-2 can be the result of:
- injury to your heart due to an accident
- inflammation of your heart muscle, which is usually from a virus
- an electrical injury
- a heart attack
Increased levels of CPK-2 in the blood can also occur after open heart surgery and heart defibrillation, which is a medical procedure that involves shocking your heart. After a heart attack, CPK-2 levels in your blood rise, but they usually fall again within 48 hours.
CPK-3 is found in your skeletal muscle. The levels of CPK-3 may rise if your muscles:
- are damaged from a crush injury, which occurs when a body part has been squeezed between two heavy objects
- have been immobile for an extended period
- are damaged by illegal drug use
- are inflamed
Other factors that cause elevated levels of CPK-3 include:
- muscular dystrophy
- muscle trauma, which can occur from participating in contact sports, being burned, or having surgery
- electromyography, which is a procedure that tests nerve and muscle function
It’s important to remember that results will vary from person to person, depending on specific injuries and conditions. Your doctor will explain what your results mean and describe your treatment options.