Creatine Kinase (CK)

Creatine kinase (CK) is an enzyme found mainly in the skeletal muscles and the heart muscle that helps supply energy to muscle cells. Normally only a small amount enters the blood, mostly from the skeletal muscles. If muscle cells are damaged, by exercise, an injury or a muscle disease, more CK enters the blood. How high the value is also depends on muscle mass, sex and training.

• Muscle damage after heavy or unusual exercise
• Muscle symptoms as a side effect of statins (cholesterol-lowering drugs)
• Muscle diseases, such as muscle inflammation (myositis) or muscular dystrophy
• Severe muscle breakdown (rhabdomyolysis)

With a raised CK, the GP asks about exercise, injuries, medicines and symptoms, and usually repeats the measurement after a period of rest. In statin users with muscle symptoms and a raised CK, the GP assesses whether the statin is temporarily stopped or replaced, according to the Dutch GP guideline (NHG) on cardiovascular risk management. If CK stays raised without explanation, further blood tests may follow (including thyroid function and kidney function) and possibly a referral to a neurologist.

The CK measurement is accurate, but a raised value in someone who exercises is hard to interpret, because exercise in the days before strongly affects the result. In statin users without muscle symptoms CK says little, because most muscle symptoms with statins do not come with a clear rise in CK and a severe rise is rare; the value is mainly informative with muscle symptoms that may point to a side effect. For diagnosing a heart attack, another marker (troponin) says more than CK. An abnormal value is therefore assessed together with symptoms, training and medicines, and repeated if in doubt after a number of days without heavy exercise.

• Heavy or unusual exercise, strength training, long-distance running and muscle injuries raise CK for up to several days after the exercise.
• Statins and some other medicines can raise CK; a normal CK does not rule out a statin side effect.
• An injection into a muscle, a fall or surgery in the days before the sample collection raise the value.
• The reference values differ by laboratory, sex and ethnic background; people with a lot of muscle mass or a dark skin color often have a higher normal value.

A raised CK after heavy exercise is very common in athletes. Muscle symptoms with statin use are reported in practice by a large share of users, but according to the Dutch medical specialists' guideline (FMS), a severe rise in CK occurs in fewer than 1 in 1,000 to 10,000 users per year. Muscle diseases such as myositis and muscular dystrophy are rare.

Useful with unexplained muscle pain, muscle weakness or dark urine, with muscle symptoms during statin use and when a muscle disease is suspected. After a period of illness with persistent muscle symptoms, CK can also help rule out muscle damage. In athletes, the result can only be interpreted if there has been no heavy training for several days. In healthy adults without symptoms, and in statin users without muscle symptoms, a raised value says little and is often caused by exercise.

• A raised CK after exercise is common and, without a repeat test after rest, can lead to unnecessary worry or unnecessary follow-up tests.
• A raised CK without symptoms can lead to unnecessarily stopping a statin, while the statin lowers the risk of cardiovascular disease.

• NVKC, allesovertesten.nl: CK
• NHG-Standaard Cardiovasculair risicomanagement (M84)
• FMS Richtlijn Erfelijke dyslipidemie in de 2e en 3e lijn: Statinegerelateerde bijwerkingen
• Thuisarts.nl: Ik ga medicijnen gebruiken tegen een hoog cholesterol