CK-MB

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
CK-MB, together with total CK, is measured when there are signs and symptoms of acute myocardial infarction (AMI). CK-MB can be detected in the blood within 6 hours of heart muscle damage. Blood levels stay high for 36-48 hours. A measurement is usually taken on arrival and again at 6-8 hours to confirm or rule out the diagnosis. Since CK can come from both heart and skeletal muscle cells, the MB isoform must be measured specifically to tell where a rise in total CK comes from. Reopening a blocked coronary artery (spontaneously or with medicines) causes a peak in the concentration of CK-MB and other serum cardiac markers.

When is it requested?
CK-MB, together with total CK, is measured when an acute myocardial infarction is suspected; the test may also be requested after high total CK values have been found, to tell whether the rise comes from the heart or from skeletal muscle.

What does the test result mean?
A rise in CK-MB or a CK-MB/total CK ratio >2.5 indicates heart damage. A rise in total CK with a ratio <2.5 points instead to damage to skeletal muscle.

Is there anything else I should know?
Rarely, damage to skeletal muscle cells can cause a significant rise in CK-MB. To distinguish between the two, troponins are measured, as they are highly specific for acute myocardial infarction.

Other possible sources of raised CK to consider for a correct differential diagnosis are: muscle diseases such as muscular dystrophy; myopathies; hypothyroidism; surgery; prolonged immobilisation

1. What is meant by a heart attack?
A “heart attack” is an acute myocardial infarction (AMI), caused by the interruption of blood flow in a coronary vessel, leading to the death of the heart muscle cells no longer supplied with blood. The interruption can be caused by a clot in a vessel already partly blocked by an atherosclerotic plaque. In rare cases AMI can be secondary to an embolism from the endocardium (e.g. infective endocarditis), congenital abnormalities or coronary spasm.

The first symptom of an AMI is usually deep, visceral pain behind the breastbone, described as tight or crushing, often spreading to the back, jaw or left arm. In a variable percentage of cases the pain can be very mild. The patient is restless and anxious, with pale, cold and sweaty skin. There may also be a sudden loss of consciousness or a state of profound weakness.

2. Does chest pain indicate an acute myocardial infarction?
Chest pain is not a specific symptom of acute myocardial infarction, so it is not possible to say that a heart attack is under way from this observation alone. Chest pain has many causes: it may come from the lungs (pneumothorax, pleurisy), the blood vessels (pulmonary embolism), the digestive system (gastritis, oesophagitis) or the nerves and joints (intercostal neuralgia, sternocostal arthritis). The diagnosis takes into account a detailed history of the pain (its quality, duration, location and whether it is linked to exertion), a thorough physical examination, an electrocardiogram and measurement of serum cardiac enzymes.

3. What other tests are done when an acute myocardial infarction is suspected?
To diagnose an acute myocardial infarction, in addition to chest pain and other symptoms, both an electrocardiogram and, in some cases, laboratory tests are needed. The most specific tests are troponins and CK-MB, even when the electrocardiogram is inconclusive. In some cases total CK and myoglobin are also measured, although these tests are less specific for acute myocardial infarction.

4. What should I do if I suspect I am having a heart attack?
If you have prolonged chest pain that does not go away with rest, you need immediate medical help; the same applies to angina when the prescribed medicines are not enough to relieve the pain.