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Specialist Advice — 5 minutes

The impact of exercise on laboratory tests: The case of creatine kinase

Raymond Lepage, PhD, Doctor in Biochemistry
Raymond Lepage, PhD, Doctor in Biochemistry
Science popularizer

Few people are aware that exercise, especially intense activity, can significantly alter certain laboratory tests. Exercise is known to contribute to increased levels of potassium, urea, creatinine, creatine kinase, lactate dehydrogenase (LDH), aspartate aminotransferase (AST), alkaline phosphatase, bilirubin, uric acid, as well as the white blood cell count. The amount of increase depends on several factors, including duration and intensity of exercise and the patient’s fitness level. One thing is clear: the less fit you are, the more vigorous and prolonged exercise may affect your results.

The role of creatine kinase

Creatine kinase, or CK, is a protein found in every muscle, including those used during exercise (striated muscles) and the ones that keep your heart beating. CK is involved in the production of energy required for muscle contraction. When muscle cells are damaged, CK, myoglobin, potassium and other substances enter the bloodstream, to eventually be broken down or eliminated by the kidneys.

What happens when you exercise?

As is well known, intense exercise often injures muscle tissue, causing CK to be released into the bloodstream. Although the increase in CK levels is usually moderate (three to five times higher than normal), an increase of up to 100 times above normal is occasionally seen in runners at the end of a marathon. Our laboratory technicians have already observed a CK level of 1,000 times higher than normal in a healthy young woman who had decided to get back in shape!

After a muscle is injured, it takes a certain amount of time for CK levels to return to normal. They should decrease by half every 36 hours. For example, a level that is 100 times higher (about 15,000 U/L) will take 10 days or so to return to normal.

Why should you limit exercise before a blood test?

Above-normal CK results may indicate a problem. A variety of conditions are likely to damage muscle cells and release CK into the bloodstream. These include muscular dystrophy, toxic reactions to certain drugs, hyperthermia (high fever) and hypothyroidism.

The cause can also be an accident or trauma where a muscle is crushed, a surgical procedure where a muscle is severed, or an intramuscular injection.

Myocardial infarction also releases CK into the blood when the tissue, including the muscles that make up the heart, dies from lack of oxygen.

When CK levels are at least 10 times higher than normal, we call this “rhabdomyolysis,” a clinical condition that can involve severe complications. Myoglobin and potassium released from damaged muscle cells are mainly responsible for this condition. Excessive myoglobin levels are toxic to the kidneys and can lead to acute renal failure, while high potassium levels can lead to heart rhythm problems.

Consequently, a high CK level may be a legitimate concern for your health care professional, who will need to determine whether it is rhabdomyolysis with risk of complications, another condition (genetic disease, heart attack, hypothyroidism, etc.) or an increase in post-exercise CK with no clinical consequences.

To make it easier to interpret your CK results, we recommend that you follow the instructions before having a blood sample taken, including advice to restrict your exercise. It may even be relevant to note the type and intensity of exercise in the days preceding the test. This reminder could be very useful when you see your health care professional again, sometimes a few weeks later, to discuss the results.

If you have any questions or need more information, please don’t hesitate to call our customer service number at 1 833 590-2712.