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Simply Stated: The Creatine Kinase Test

Almost everyone with a neuromuscular disorder has had, or will have, a creatine kinase test. But what exactly is creatine kinase (CK), and why are its levels measured in neuromuscular diseases?

CK, also known as phosphocreatine kinase, or CPK, is a type of protein called an enzyme. It catalyzes, or "encourages," a biochemical reaction to occur. The normal function of CK in our cells is to add a phosphate group to creatine, turning it into the high-energy molecule phosphocreatine. Phosphocreatine is burned as a quick source of energy by our cells.

However, the normal function of CK isn't as relevant, in this case, as what happens to CK when muscle is damaged. During the process of muscle degeneration, muscle cells break open and their contents find their way into the bloodstream. Because most of the CK in the body normally exists in muscle, a rise in the amount of CK in the blood indicates that muscle damage has occurred, or is occurring.

To measure CK levels, a blood sample is taken and separated into fractions that contain cells and a fraction that doesn't — the serum. The amount of CK in the serum is reported in units (U) of enzyme activity per liter (L) of serum. In a healthy adult, the serum CK level varies with a number of factors (gender, race and activity), but normal range is 22 to 198 U/L (units per liter).

Higher amounts of serum CK can indicate muscle damage due to chronic disease or acute muscle injury. For this reason, if you're scheduled to have blood drawn for a CK test to diagnose a potential muscle disorder, you should limit your exercise to normal activities before the test.

CK tests are used to evaluate neuromuscular diseases in five basic ways:

  1. To confirm a suspected muscle problem before other symptoms occur
  2. To determine whether symptoms of muscle weakness are caused by a muscle or a nerve problem
  3. To differentiate between some types of disorders such as dystrophies versus congenital myopathies
  4. To detect "carriers" of neuromuscular disorders, particularly in Duchenne muscular dystrophy. A carrier has a genetic defect, but doesn't get the full-blown disease. A carrier's child may have the full disease.
  5. To follow the course of a disease that fluctuates (primarily the inflammatory myopathies), or to document episodes of acute muscle injury, as might occur in some metabolic myopathies.

Because elevated CK levels indicate muscle damage, many parents wonder why their children with Duchenne muscular dystrophy (DMD) had higher CK levels when they were younger and had more muscle function. This seeming paradox occurs because muscle degeneration is more rapid at the earlier stages and, possibly, because there's more muscle bulk available to release CK into the circulation at this time.

CK levels can be slightly elevated (500 U/L) in nerve disorders like Charcot-Marie-Tooth disease, amyotrophic lateral sclerosis or spinal muscular atrophy, or grossly elevated (3,000 to 3,500 U/L) in DMD or inflammatory myopathies.

During episodes of acute muscle breakdown (rhabdomyolysis), CK levels can temporarily go off the scale, topping out at 50,000 to 200,000 U/L. At the same time, some neuromuscular disorders, such as the congenital myopathies (nemaline, central core disease and others) and myasthenia gravis, may not trigger any elevation of CK levels. CK levels don't always reflect the level of functional impact on the individual.

"Simply Stated" is a Quest column designed to explain some terms and basic facts about neuromuscular disease.

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