Creatine phosphokinase (CPK), sometimes called CK, is an enzyme primarily found in the brain, lungs, heart, and skeletal muscles of the body. Like all enzymes, it is a protein that facilitates various chemical processes in the body. CPK acts as a catalyst in the phosphorylation of muscle creatine.
As it is mainly stored in the muscles of the body, any trauma or injury to the muscles, including myocardial infarction (MI) or heart attack, can cause the CPK to leak into the bloodstream. Therefore, if a blood test reveals elevated levels of CPK, it may indicate a heart attack.
Since CPK is stored in other muscles and body organs as well, the doctor may require CPK (isoenzyme) testing to find out the type of CPK test that is elevated. Other conditions that could cause CPK elevation include muscular dystrophy, stroke, pulmonary infarction, inflammation of muscles, electric shock, etc. But first, let us take a look at the type of CPK elevation that will indicate heart damage.
Why is the CPK test relevant?
Depending on their location, the Creatinine Phosphokinase enzyme is classified into three types -
- CPK-1 or CPK-BB, found in the brain and lungs
- CPK-2 or CPK-MB, found in the heart
- CPK-3 or CPK-MM, found in skeletal muscle
Of these, the CPK-2 or the CPK-MB is one of the key diagnostic tools that help doctors determine the patient's risk of suffering a heart attack or assessing the extent of damage to the heart tissue if the heart attack has already occurred.
How accurate is the CPK-MB test in diagnosing MI?
Creatine phosphokinase-MB is by far the most reliable diagnostic indicator for an acute heart attack or MI. It is a highly specific and very sensitive test and is also very commonly available. CPK-MB levels rise within 6 hours of a heart attack but start to fall back to normal within 48 hours. It is invaluable in monitoring the patient's condition following an acute MI. Despite these advantages, CPK results must be interpreted by the doctor after considering several factors.
A patient may have elevated CPK levels following any injury to the heart muscle or after open-heart surgery. In addition, if the patient has required defibrillation recently, chances are his/her CPK levels will be elevated. Viral myocarditis, or a viral infection causing heart muscle inflammation, may also lead to elevated CPK levels. There are certain cases, though, in which CPK may be elevated without accompanying heart disease. These include –
- Following vigorous exercise
- Use of narcotic drugs
- Use of medicines such as steroids, statins, amphotericin B, etc.
- Following cardiac catheterisation
- Following a surgical intervention
Since CPK elevation may also occur in other cardiac injuries or inflammatory disorders, the value of the test in diagnosing and monitoring MI has been under debate. It is because MI can be serious and turn fatal without timely intervention. Despite these challenges, CPK is a highly effective tool in offering immediate indications of cardiac disease. Combined with other blood tests like cardiac troponin, it is definitive in diagnosing MI. Providing your doctor with a complete and accurate medical history and list of symptoms is important for reaching an accurate diagnosis.
Symptoms of MI
Heart attack or Myocardial Infarction can start slowly or come on suddenly. It is important to recognise the symptoms and seek medical attention immediately. Some of the symptoms of a heart attack include -
- Chest pain or discomfort - Sudden pain or pain that comes and goes could be signs of a heart attack. Sometimes the discomfort can feel like pressure or a squeezing grip.
- Pain in the jaw or left arm
- Breathlessness or respiratory distress with profuse sweating
- Nausea
- Light-headedness or dizziness
- Loss of consciousness
If the doctor suspects that a patient may be experiencing a heart attack, it is likely that the patient will be admitted to an intensive care facility and monitored closely. Apart from CPK, other tests that the doctor may recommend include -
- Troponin-I (Cardiac Troponin)
- Electrocardiogram (ECG)
- Chest X-Ray
- Echocardiogram
- Coronary angiography
The CPK Test
CPK-MB is tested by drawing a vial of your blood. The phlebotomist will draw your blood through a tiny needle prick in a vein of your forearm. The pain and discomfort due to the sting of the needle are minimal and do not usually cause a lot of pain. Therefore, this test does not require any prior preparation. Like all blood tests, staying well hydrated is recommended to make it easier for the phlebotomist to locate a viable vein. It is also recommended that you wear clothes with loose sleeves. If you suffer from bleeding disorders or take blood thinning medication, you must inform the phlebotomist before the test. The laboratory may monitor you for a few minutes after the test before you can leave.
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