Clinical Biochemistry & Cardiology & Muscle Neurology
CK - Creatine Kinase
Creatine kinase (CK)—formerly known as creatine phosphokinase or CPK—is a cytosolic and mitochondrial enzyme whose primary function is the reversible transfer of a phosphate group from phosphocreatine to ADP, thereby rapidly regenerating ATP in tissues with high energy demands—primarily skeletal muscle, the myocardium, and the brain. The reaction catalyzed is: phosphocreatine + ADP ⇌ creatine + ATP. This phosphocreatine/creatine kinase system constitutes the primary energy reserve mobilized during intense muscle contraction, before anaerobic glycolysis and oxidative phosphorylation take over. CK is released into the bloodstream during any injury or overload of muscle cells—it is therefore a highly sensitive marker of muscle tissue damage. Its plasma measurement is widely used in clinical practice to assess muscle damage in numerous situations: diagnosis and monitoring of inflammatory and hereditary myopathies, monitoring of statin-induced muscle toxicity, diagnosis of rhabdomyolysis, evaluation of asymptomatic enzyme elevation (hyperCKemia), and historically for the diagnosis of myocardial infarction prior to the advent of highly sensitive troponin. CK is a homodimer or heterodimer of two subunits—M (muscle) and B (brain)—giving rise to three isoenzymes with distinct tissue distributions: CK-MM (skeletal muscle—95–98% of total CK in the blood), CK-MB (myocardium—useful for diagnosing AMI before the era of high-sensitivity troponin), and CK-BB (brain—rarely measured routinely because it does not cross the blood-brain barrier in significant quantities).
Reference values
| Population | Normal values | Remarks |
|---|---|---|
| Adult male | 40–320 U/L | Normal values are higher in men due to greater muscle mass; regular physical activity and Black race are associated with higher baseline values. |
| Adult woman | 25-200 U/L | Lower than human values; pregnancy may slightly alter normal values depending on the trimester |
| Newborn and infant | 70–380 U/L | CK physiologically more elevated at birth (obstetric labor, muscular immaturity); normalizes in the first weeks; very high value in the newborn suggests congenital myopathy |
| Child (1-12 years) | 60-300 U/L | Slightly higher than adult; very high CK in a boy (>10x N) → mandatory Duchenne myopathy screening |
| Elderly person (> 70 years old) | 30–200 IU/L | Generally lower values due to sarcopenia (reduction in muscle mass); low normal CK in a very elderly subject does not indicate good muscle health |
| People of African descent | Up to 2x usual LSN | Physiologically higher baseline values — genetic mechanism not fully elucidated (possibly related to muscle fiber composition); to be taken into account before attributing asymptomatic hyperCKemia to a pathology |
CK Isoenzymes — Distribution and Clinical Utility
- CK-MM (M-M dimer): accounts for 95–98% of total circulating CK in healthy individuals; produced exclusively by skeletal muscle; the predominant fraction in all cases of muscle damage (rhabdomyolysis, myopathy, trauma, intense exercise); elevated levels accurately reflect overall muscle damage; no diagnostic value per se—it is the elevation of total CK that provides the key
- CK-MB (M-B dimer): accounts for 1 to 4% of total CK in normal blood; produced primarily by the myocardium (15–25% of cardiac CK is CK-MB) and to a small extent by regenerating skeletal muscle (myosatellites, smooth muscle); historically the gold standard marker for myocardial infarction (MI)—sensitivity 50–60% at 4 hours, 80–90% at 8 hours, peak at 18–24 hours, normalization at 48–72 hours; now largely replaced by ultra-sensitive troponin I or T (TnUs) for the diagnosis of MI (sensitivity 97–99% at 3 hours with the 0h/3h algorithm); CK-MB remains useful for detecting early reinfarction (return to normal followed by a rise after the initial MI—TnUs remains elevated longer) and for assessing the size of the necrotic area (area under the CK-MB curve correlates with the volume of necrotic myocardium); CK-MB/total CK ratio > 6 at % — suggestive of myocardial injury (vs. pure skeletal muscle injury)
- CK-BB (B-B dimer): produced primarily by the brain and smooth muscles (uterus, prostate, intestine); does not normally cross the blood-brain barrier—undetectable in normal blood; elevated CK-BB in blood → severe stroke, severe head trauma, cerebral anoxia, vascular surgery, certain cancers (prostate cancer, breast cancer); rarely measured routinely—can be a source of unexplained elevation in total CK in certain cases of occult malignancy
- Mitochondrial CK (CK-Mt): an isoenzyme located in the inner mitochondrial membrane — accounts for < 1% of total CK in normal blood; elevated levels are associated with mitochondrial diseases, severe myopathies, and certain cancers (a marker of poor prognosis); this test is rarely performed in routine clinical practice
- Macro-CK: CK complex (often CK-BB) bound to immunoglobulins (IgG or IgM) → large molecule with low clearance → chronically elevated total CK without muscle or cardiac pathology; cause of unexplained elevated CK in 1–2% of hospitalized patients; detected by isoenzyme electrophoresis or polyethylene glycol precipitation; benign but may lead to numerous unnecessary investigations if unrecognized
Causes of elevated CK by amplitude
| Cause | Amplitude CK | Mechanism and characteristics |
|---|---|---|
| Intense physical exercise | 2–20 x N | Micro-lesions of muscle fibers during eccentric exercise (downhill running, weightlifting, marathon) - peak at 24-72 hours after exertion, normalization in 5-7 days; correlates with intensity and muscle non-habituation (DOMS - delayed onset muscle soreness); not to be confused with muscle pathology - always measure CK at a distance from any intense exercise (> 72 hours of rest) |
| Intramuscular injection | 2–5x N | Direct trauma to muscle fibers by the needle and injected product — elevation proportional to the injected volume and depth; fasting before any injection for a reliable baseline CK |
| Inflammatory myopathies (polymyositis, dermatomyositis, autoimmune necrotizing myopathy) | 5–50× N (or more) | Autoimmune destruction of skeletal muscle fibers → very high CK (sometimes > 10,000 U/L in necrotizing myositis with anti-SRP or anti-HMGCR antibodies); symmetric proximal muscle weakness (shoulders, hips); myopathic EMG + muscle biopsy (perivascular lymphocytic infiltrate in PM, perifascicular in DM); specific myositis antibodies (anti-Jo-1, anti-Mi-2, anti-SRP, anti-MDA5, anti-TIF1); CK correlates with inflammatory activity and response to treatment (corticosteroids, azathioprine, IVIG, rituximab). |
| Hereditary myopathies (Duchenne, Becker, LGMD, dystrophin) | 10–200× N | Duchenne muscular dystrophy (DMD) — DMD gene mutation (Xp21) → absence of dystrophin → muscle fiber membrane instability → chronic necrosis → very high CK from birth (often > 10,000–50,000 U/L) before clinical signs appear; newborn screening in development in Quebec; Becker muscular dystrophy (BMD) — milder allelic form, high CK but less than Duchenne; LGMD (limb-girdle muscular dystrophies) — various mutations (calpain, dysferlin, gamma-sarcoglycan) → moderately to very high CK depending on the type |
| Statin myopathy | Myalgia: 10x normal | Mechanism: CoQ10 depletion, mitochondrial dysfunction, reduced membrane cholesterol in muscle fibers; frequency as shown in the table — myalgia (pain without elevated CK): 5–10 % (often nocebo — SAMSON trial 2020); myositis (CK 4–10× ULN + symptoms): 0.5 %; rhabdomyolysis (> 10× ULN + myoglobinuria): 0.01 %; risk factors: high dose, advanced age, hypothyroidism, chronic kidney disease (CKD), drug interactions (fibrates—gemfibrozil ++, amiodarone, cyclosporine, CYP3A4 inhibitors for statins metabolized by this CYP); discontinue statin if CK > 10× ULN or if CK 4–10× ULN with disabling symptoms |
| Rhabdomyolysis | 1,000–5,000x N | Massive destruction of skeletal muscle fibers → release of myoglobin, CK, potassium, phosphorus, and uric acid into the circulation → acute kidney injury (myoglobin precipitates in the renal tubules); CK often > 100,000 U/L — may reach several million U/L; main causes: crush injury (compartment syndrome), prolonged seizures, status epilepticus, prolonged immobilization (forced decubitus position), malignant or exercise-induced hyperthermia, heatstroke, intoxications (alcohol, cocaine, heroin, amphetamines), severe viral infections (influenza, severe COVID-19), severe electrolyte abnormalities (hypokalemia, hypophosphatemia); treatment: aggressive IV hyperhydration (0.9% NaCl 1–3 L/h or Ringer’s lactate 1–1.5 L/h) to protect the renal tubules, urine alkalinization if urinary pH < 6.5 (IV bicarbonate), hourly urine output monitoring (target ≥ 1–3 mL/kg/h), creatinine monitoring, serum potassium (hyperkalemia → fatal arrhythmia) |
| Myocardial infarction (MI) | 3-20 x N (Total CK) | Myocardial necrosis → release of CK-MB and total CK; kinetics: rise begins at 4–6 hours, peaks at 18–24 hours, returns to normal at 48–72 hours; CK-MB/total CK ratio > 6 % + clinical context (chest pain, ECG); ultrasensitive troponin (TnUs) is now the gold standard marker—much more sensitive and specific than CK-MB for the early detection of acute myocardial infarction |
| Hypothyroidism | 2–10 × N | Hypothyroidism reduces clearance and increases muscle production of CK; a common and easily overlooked cause of chronically elevated CK—TSH should be systematically measured in the presence of any unexplained elevation of CK; normalization of CK after correction of hypothyroidism with levothyroxine (4 to 12 weeks delay) |
| Muscle trauma, surgery, electrocution | 10–1000x N depending on severity | Direct mechanical injury to muscle fibers; major orthopedic surgery (total hip/knee replacement → elevated CK 24–48h post-op); external electrical cardioversion → elevated CK (especially CK-MM) without significant CK-MB; severe electrocution → possible rhabdomyolysis |
| Mitochondrial diseases | 2–10 × N | Mitochondrial dysfunction → disruption of muscle fiber energy metabolism → moderately elevated CK; context: profound fatigue, muscle weakness, ptosis, ophthalmoplegia, deafness, diabetes, cardiomyopathy — multisystemic clinical presentation; muscle biopsy (ragged red fibers on modified Gomori stain) + mitochondrial and nuclear DNA sequencing |
Isolated asymptomatic hyperkalemia
- Definition: Persistently elevated total CK levels (>1.5 times the upper limit of normal on at least two tests performed 4 weeks apart) without muscle weakness, significant muscle pain, or an obvious cause; estimated prevalence of 1–2 per 1,000 in the general adult population
- Recommended workup for asymptomatic hyperCKemia: measure CK at least 72 hours after intense exercise, after at least one night of rest, and without recent intramuscular injections; TSH (hypothyroidism is very common and treatable); creatinine and eGFR (chronic kidney disease); electrolytes (hypokalemia, hypophosphatemia); LDH, AST (associated liver pathology); comprehensive myositis workup (anti-Jo-1, anti-SRP, anti-HMGCR—especially if CK > 5x normal); complete neurological and muscular clinical examination; EMG if weakness or persistently elevated CK (> 5–10x normal); muscle biopsy if indicated by workup (myopathic EMG, positive antibodies, suspected muscular dystrophy).
- Genetic causes of asymptomatic hyperCKemia: carrier of the Duchenne muscular dystrophy gene (female DMD carrier—elevated CK in 50–70% of cases); asymptomatic Becker muscular dystrophy (mild allelic form); early-onset dysferlinopathy (LGMD type 2B — may present with asymptomatic hyperCKemia 5–15 years before muscle weakness); certain heterozygous carriers of calpain-3 mutations
- Macro-CK: to consider if CK is persistently elevated (moderately, often 200-1,000 U/L) without any symptoms and without other biological abnormalities — isoenzyme measurement by electrophoresis or PEG precipitation allows detection; benign, does not require treatment; avoids multiple costly and anxiety-inducing muscle investigations
ℹ️
CK must always be measured under standardized conditions: fasting (or at least 4 hours without a high-fat meal), after at least 72 hours without intense physical exercise, without recent intramuscular injection, and without prolonged pressure on the muscles (immobilization). A single high reading after intense exertion or a fall has no diagnostic value; repeating the measurement after a rest period is essential before initiating a workup for myopathy.
Rhabdomyolysis — Medical Emergency
Rhabdomyolysis is a medical emergency characterized by massive destruction of muscle fibers and the release of myoglobin into the blood and urine. Warning signs include: reddish-brown or «black tea» colored urine (myoglobinuria), intense and diffuse muscle pain, severe muscle weakness, oliguria, or anuria. Compose the 911 You should go to the emergency room immediately if rhabdomyolysis is suspected. The main risks are acute kidney failure requiring dialysis and hyperkalemia, which can cause fatal cardiac arrhythmias. A CK above 10,000–20,000 U/L with elevated creatinine should be considered rhabdomyolysis until proven otherwise and treated in a hospital setting with intensive intravenous hydration.
Kinetics of CK in myocardial infarction - comparison with troponin
| Marker | Kinetics in IDM | Current usefulness |
|---|---|---|
| Total CK | Onset of elevation: 4–6h; peak: 24h; normalization: 72h | Rarely used for diagnosing MI — replaced by troponin; useful for estimating infarct size (area under the curve) |
| CK-MB | Onset of elevation: 3–4 h; Peak: 18–24 h; Normalization: 48–72 h | Also useful for detecting an early recurrent myocardial infarction (return to normal followed by a rise); a CK-MB/CK ratio > 6 (1-fold to 3-fold) suggests a cardiac cause |
| Ultrasensitive Troponin I or T (UsTn) | Onset of elevation: 1–2 hours; peak: 12–24 hours; normalization: 5–14 days | IDM reference marker (sensitivity 97–99% at 1 day post-onset, specificity 90–95% at 1 day post-onset) — 0-hour/1-hour or 0-hour/3-hour algorithm; replaces CK and CK-MB for initial diagnosis |
Management by CK Level
- CK 72h without intense exercise, without MI); dose TSH, creatinine, serum potassium, current medication list; if persistent → further targeted workup
- CK 4-10x N: More in-depth assessment — EMG, myositis antibodies, muscle MRI if indicated; neuromuscular evaluation; if currently on statins → temporary discontinuation and re-evaluation at 6 weeks
- CK > 10x N (without overt rhabdomyolysis): urgently rule out rhabdomyolysis (creatinine, electrolytes, urine dipstick for myoglobinuria); if rhabdomyolysis, then hospitalization and IV hyperhydration; if stable, then complete myopathy workup (EMG, antibodies, MRI, muscle biopsy) via specialist consultation
- CK > 100,000 U/L: severe rhabdomyolysis → absolute hospital emergency; risk of acute kidney injury, fatal hyperkalemia, coagulation disorders (DIC), compartment syndrome; monitoring in ICU or intensive care unit
Consult at Clinique Omicron
Clinique Omicron's doctors assess elevated CK levels within their clinical context (medications, history, muscle signs), prescribe targeted investigations (TSH, ionogram, myositis antibodies, EMG), adjust statin treatments, and refer to neurologists or rheumatologists for the management of identified myopathies. Consultations are available at our service points in Quebec and via telemedicine. To book an appointment, visit cliniqueomicron.ca.
The content of this page is for informational purposes only and does not substitute for advice from a qualified healthcare professional. CK reference values vary by laboratory — always refer to the standards of the laboratory that performed the analysis.
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