Creatine Kinase (CK) Unit Converter
Creatine Kinase (CK) Unit Converter
Convert creatine kinase between U/L, IU/L, µkat/L and nkat/L, with the statin and rhabdomyolysis thresholds and why one reference interval does not fit everyone.
Creatine Kinase (CK) converter
Activity unitsCreatine kinase 180 U/L
Formula and conversion factor
U/L = µkat/L × 60
nkat/L = U/L × 16.667
- U
- 1 unit = 1 µmol of substrate converted per minute
- kat
- 1 katal = 1 mol of substrate converted per second, so 1 U/L = 1/60 µkat/L
- IU/L
- numerically identical to U/L
- reference interval
- sex-specific, and substantially higher in people of African ancestry; the interval used changes whether a result is called abnormal
Worked example
Creatine kinase 180 U/L
180 ÷ 60 = 3.000 µkat/L
= 3,000 nkat/L = 180 IU/L
Within the adult male reference interval of 40–320 U/L, and below the female upper limit of 200 U/L
Reference intervals and thresholds in both conventions
| U/L | µkat/L | |
|---|---|---|
| Adult men, reference interval | 40 – 320 | 0.67 – 5.33 |
| Adult women, reference interval | 25 – 200 | 0.42 – 3.33 |
| Statin myopathy — usual threshold for stopping, with symptoms | > 10 × upper limit | > 10 × upper limit |
| Conventional rhabdomyolysis threshold | > 5,000 | > 83 |
Working through a raised CK
| Consider first | Then | Finally |
|---|---|---|
| Exercise in the past few days, especially unaccustomed or eccentric | Statins and other myotoxic drugs, thyroid disease, alcohol | Inflammatory myopathy, inherited myopathy, muscular dystrophy carrier state |
| Intramuscular injection, fall, seizure, prolonged lie | Hypothyroidism, which raises CK and is easily missed | Persistent unexplained elevation — refer rather than repeat |
| Race and sex of the patient against the interval used | Repeat after several days without exercise | Consider genetic myopathy panels and neurology assessment |
Why the history matters more than the number
Creatine kinase is reported as catalytic activity, in U/L or the numerically identical IU/L, and in µkat/L or nkat/L where SI units are used. Since a unit is one micromole per minute and a katal is one mole per second, the conversion is a division by 60 and nothing else: a CK of 180 U/L is 3.000 µkat/L. The isoenzyme fractions, CK-MB and CK-MM, use the same units.
The commonest causes of a raised CK are entirely benign. Unaccustomed or eccentric exercise raises it for several days and can push it into the thousands in a healthy person; an intramuscular injection, a fall, a seizure or a long lie on the floor will do the same. Taking a history and repeating the test after several days of rest resolves most raised results without any further investigation, and doing so before ordering a myositis panel avoids a great deal of unnecessary work.
Which reference interval is applied changes the answer. Intervals are sex-specific, with a lower upper limit in women, and they are substantially higher in people of African ancestry — enough that applying a single generic interval labels healthy Black patients as abnormal and generates repeated investigation with nothing at the end of it. Hypothyroidism is the other easily missed cause, and thyroid function is worth checking early in anyone with an unexplained elevation.
Two thresholds recur in practice. In suspected statin myopathy, a level above ten times the upper limit of normal in a patient with muscle symptoms is the usual trigger for stopping the drug; below that, symptoms without a marked rise are common and do not by themselves require withdrawal. Rhabdomyolysis is conventionally considered above about 5,000 U/L, with the risk of acute kidney injury rising as the level does — though that relationship is continuous rather than threshold-based, and fluid resuscitation should not wait for a number. A persistently raised CK with no explanation deserves a search for an inherited myopathy rather than repeated reassurance.
Frequently asked questions
How do I convert creatine kinase from U/L to µkat/L?
Divide by 60, because a unit is one micromole per minute and a katal is one mole per second. A CK of 180 U/L is 3.000 µkat/L, or 3,000 nkat/L. IU/L and U/L are the same number.
What is the commonest cause of a raised CK?
Recent exercise, particularly if unaccustomed or eccentric, which can raise CK into the thousands in a healthy person for several days. An intramuscular injection, a fall, a seizure or a prolonged lie will also do it. Repeating after several days of rest resolves most cases.
Do CK reference intervals differ between people?
Yes. They are sex-specific, with a lower upper limit in women, and they are substantially higher in people of African ancestry. Applying one generic interval to everyone labels healthy people as abnormal and generates avoidable investigation.
When should a statin be stopped for a raised CK?
The usual threshold is a level above ten times the upper limit of normal in a patient with muscle symptoms. Muscle symptoms with a normal or mildly raised CK are common and do not in themselves require withdrawal, though they may still warrant a change of drug or dose.
What CK level defines rhabdomyolysis?
Conventionally about 5,000 U/L, roughly 83 µkat/L, but this is a convention rather than a physiological cut-off. The risk of acute kidney injury increases continuously with the level, and fluid resuscitation should be guided by the clinical picture rather than by crossing a number.
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References
- Stroes ES, Thompson PD, Corsini A, et al. Statin-associated muscle symptoms: impact on statin therapy — European Atherosclerosis Society Consensus Panel Statement. Eur Heart J. 2015;36(17):1012–1022.
- Bosch X, Poch E, Grau JM. Rhabdomyolysis and acute kidney injury. N Engl J Med. 2009;361(1):62–72.
- IFCC reference procedure for the measurement of catalytic activity concentration of creatine kinase at 37°C.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.