Biochemistry · Topic
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)
- Acute compartment syndrome
- McArdle disease (glycogen storage disease type V)
- Carnitine palmitoyltransferase II (CPT-II) deficiency
- Hypothyroidism
Related
- Cardiac troponin as a lab test
- Molecules derived from amino acids
- ATP (adenosine triphosphate)
- Myoglobin
- Creatinine and estimated glomerular filtration rate (eGFR)
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.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Creatine Kinase: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Creatine phosphokinase test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); CPK isoenzymes test: MedlinePlus Medical Encyclopedia (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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Representative model, for educational purposes only.