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# Creatine kinase (CK) and CK-MB as lab tests

CK leaks from damaged muscle; it is the key marker of rhabdomyolysis, and its MB fraction was used for heart attack before troponin.

Updated 2026-11-01

## Key facts

- It catalyses creatine + ATP ⇌ phosphocreatine + ADP: the fast energy reserve of muscle and brain.
- Isoenzymes (M and B dimers): CK-MM (skeletal muscle), CK-MB (myocardium, also some in muscle), CK-BB (brain, gut, prostate).
- Rhabdomyolysis gives very high CK, myoglobinuria (dark urine, dipstick positive for blood without red cells) and a risk of acute kidney injury and hyperkalaemia.
- Troponin replaced CK-MB for diagnosing myocardial infarction; CK-MB returns to normal sooner (2–3 days) and helps detect reinfarction.
- Values vary widely with muscle mass, sex, ancestry and exercise: each laboratory sets its own range.

## In detail

- What it measures: total CK activity in serum; CK-MB is measured as mass by immunoassay.
- Where it comes from: skeletal muscle supplies nearly all serum CK; the myocardium supplies the largest share of CK-MB.
- Why it rises: trauma and crush, exhausting exercise, seizures, intramuscular injections, statins (myopathy), hypothyroidism, inflammatory myopathies, muscular dystrophies (Duchenne), malignant hyperthermia, neuroleptic malignant syndrome, muscle glycogenoses (McArdle) and CPT II deficiency.
- Pitfalls: very muscular people and people of African ancestry have a higher baseline CK; macro-CK (CK bound to immunoglobulin) raises CK without disease.
- In rhabdomyolysis, filtered myoglobin precipitates in the tubule and its haem generates free radicals; early fluids aim to prevent kidney injury.

**Also called**
CK, CPK, Creatine phosphokinase, CK-MB

## Clinical notes

- A patient with muscle pain, tea-coloured urine and CK in the thousands after exertion or a crush has rhabdomyolysis until proven otherwise.
- CK is checked in people on statins who develop muscle pain or weakness.
- In compartment syndrome CK rises, but the diagnosis is clinical and by compartment pressure.

## Related conditions

- [Myocardial Infarction (Heart Attack)](https://durgical.com/en/conditions/myocardial-infarction/)
- [Acute compartment syndrome](https://durgical.com/en/conditions/acute-compartment-syndrome/)
- [McArdle disease (glycogen storage disease type V)](https://durgical.com/en/conditions/mcardle-disease/)
- [Carnitine palmitoyltransferase II (CPT-II) deficiency](https://durgical.com/en/conditions/carnitine-palmitoyltransferase-ii-deficiency/)
- [Hypothyroidism](https://durgical.com/en/conditions/hypothyroidism/)

## Related

- [Cardiac troponin as a lab test](https://durgical.com/en/biochemistry/topics/cardiac-troponin-as-a-lab-test/)
- [Molecules derived from amino acids](https://durgical.com/en/biochemistry/topics/molecules-derived-from-amino-acids/)
- [ATP (adenosine triphosphate)](https://durgical.com/en/biochemistry/molecules/atp/)
- [Myoglobin](https://durgical.com/en/biochemistry/enzymes/myoglobin/)
- [Creatinine and estimated glomerular filtration rate (eGFR)](https://durgical.com/en/biochemistry/topics/creatinine-and-estimated-glomerular-filtration-rate/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which reaction does creatine kinase catalyse?

Creatine + ATP ⇌ phosphocreatine + ADP; phosphocreatine is muscle's immediate energy reserve.

## Which isoenzyme predominates in the myocardium?

CK-MB, although the myocardium also has CK-MM; so CK-MB is not fully heart-specific.

## Which urine finding suggests rhabdomyolysis?

A dipstick positive for blood with no red cells under the microscope, because it detects myoglobin.

## Why is CK-MB still useful when troponin is available?

It normalises in 2–3 days, before troponin, so a new rise helps recognise a reinfarction.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Creatine Kinase: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/creatine-kinase/) (MedlinePlus, US National Library of Medicine, 2026); [Creatine phosphokinase test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003503.htm) (MedlinePlus, US National Library of Medicine, 2026); [CPK isoenzymes test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003504.htm) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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