Unit Converter
Creatine Kinase (CK)
(Total CK – Marker of Muscle Injury, Myocardial Damage & Rhabdomyolysis)
Synonyms
- CK
- Creatine kinase
- CPK (older term)
- Total CK
- Creatine phosphokinase
- ATP:creatine phosphotransferase
Units of Measurement
- nkat/L
- µkat/L
- nmol/(s·L)
- µmol/(s·L)
- U/L
- IU/L
- µmol/(min·L)
- µmol/(h·L)
- µmol/(h·mL)
Description
Creatine kinase (CK) is a cytosolic enzyme involved in energy metabolism of muscle cells. It catalyzes:
Creatine phosphate+ADP↔Creatine+ATP\text{Creatine phosphate} + \text{ADP} \leftrightarrow \text{Creatine} + \text{ATP}Creatine phosphate+ADP↔Creatine+ATP
CK is abundant in:
- Skeletal muscle (CK-MM)
- Cardiac muscle (CK-MB)
- Brain (CK-BB)
Total CK is a sensitive marker of muscle injury, but not specific to any one tissue.
Physiological Role
- Regenerates ATP during muscle contraction
- Maintains cellular energy stores
- Highly expressed in metabolically active tissues
Clinical Significance
High CK (Hyper-CK-emia)
1. Skeletal Muscle Injury (Most Common)
- Rhabdomyolysis (CK often >5000–10,000 U/L)
- Trauma / crush injury
- Strenuous exercise
- Seizures
- Myositis (viral, autoimmune)
- Muscular dystrophies (DMD, BMD)
- Drug-induced: statins, fibrates, antipsychotics
2. Myocardial Infarction (Historically)
CK-MB preferred over total CK - troponins now gold standard.
3. Hypothyroidism
Can elevate CK significantly.
4. Renal Failure
CK rises due to decreased clearance and muscle injury.
5. Infections
Myositis (influenza, HIV, COVID) → high CK.
Low CK
Rare and usually not clinically significant.
Seen in:
- Aging
- Low muscle mass
- Chronic steroid use
Low CK values generally not interpreted diagnostically.
Reference Intervals
(Tietz 8E + IFCC + Mayo + ARUP)
Ranges vary by age, sex, race, and muscle mass.
Adult Reference Ranges (Typical)
- Men: 40 – 200 U/L
- Women: 30 – 170 U/L
Children
Higher levels due to increased muscle turnover.
Critical Values
- CK > 5000–10,000 U/L → risk of AKI (rhabdomyolysis)
- CK > 20,000–50,000 U/L → severe rhabdomyolysis
Units Description & Conversion Factors
CK measurements reflect ENZYME ACTIVITY, not mass.**
1 katal (kat) = 1 mol/s of enzymatic activity
Summary Table
| Unit | Meaning |
| U/L or IU/L | micromoles per minute per liter |
| nkat/L | nanokatals per liter |
| µkat/L | microkatals per liter |
| nmol/(s·L) | nanomoles per second per liter |
| µmol/(s·L) | micromoles per second per liter |
| µmol/(min·L) | micromoles per minute per liter |
| µmol/(h·L) | micromoles per hour per liter |
| µmol/(h·mL) | micromoles per hour per milliliter |
Diagnostic Uses
1. Rhabdomyolysis
- CK rises within 2–12 hours after muscle injury
- Peaks at 24–72 hours
- Declines over 3–7 days
- Essential for monitoring severity
2. Neuromuscular Disorders
Elevated CK seen in:
- Duchenne/Becker muscular dystrophy
- Polymyositis / dermatomyositis
- Myotonic dystrophy
3. Myocardial Injury (Historical Use)
CK-MB now preferred for cardiac evaluation.
4. Hypothyroidism
High CK is a classic finding.
5. Drug or Toxin Exposure
Monitor CK in:
- Statin therapy
- Alcohol use
- Illicit drugs (cocaine, amphetamines)
6. Exercise Physiology
CK tracks skeletal muscle stress.
Analytical Notes
- Serum preferred
- Hemolysis increases CK slightly
- Avoid IM injections prior to testing (may raise CK)
- Macro-CK (bound CK) may falsely elevate values
- IFCC standardized kinetic rate assays used in modern analyzers
Clinical Pearls
- CK >10,000 U/L strongly suggests rhabdomyolysis → aggressive IV fluids required.
- Statin-induced myopathy: CK 10× upper limit + muscle symptoms.
- CK-MB fraction helps differentiate cardiac vs skeletal source when needed.
- Hypothyroidism is a treatable cause of chronic CK elevation.
- CK can remain normal in some muscular dystrophies in late disease due to muscle loss.
Interesting Fact
Creatine kinase is one of the first enzymes released during muscle injury and can increase over 100-fold in severe rhabdomyolysis.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Enzymes.
- IFCC Reference Methods for CK Measurement.
- Mayo Clinic Laboratories - CK.
- ARUP Consult - Muscle Disease Evaluation.
- AAN Neuromuscular Disorder Guidelines.