Unit Converter
Creatine Kinase-MB (activity)
(CK-MB Enzymatic Activity – Cardiac Isoenzyme for Myocardial Injury Assessment)
Synonyms
- CK-MB (activity)
- CK-MB enzymatic activity
- MB isoenzyme of creatine kinase
- Creatine phosphokinase-MB (activity)
- CK-2 activity
(Note: CK-MB mass is more specific, but CK-MB activity is still used in some labs.)
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
CK-MB is the cardiac-specific isoenzyme form of creatine kinase (CK). It catalyzes:
Creatine phosphate+ADP↔Creatine+ATP\text{Creatine phosphate} + \text{ADP} \leftrightarrow \text{Creatine} + \text{ATP}Creatine phosphate+ADP↔Creatine+ATP
CK-MB activity reflects enzymatic rate, not concentration.
Although cardiac troponins are now the gold standard, CK-MB activity assays remain useful in:
- Early myocardial infarction (MI) detection
- Reinfarction detection
- Institutions without troponin availability
- Situations where troponin is chronically elevated (CKD, myocarditis)
CK Isoenzymes
- CK-MM → skeletal muscle
- CK-MB → heart (15–40% of CK in myocardium)
- CK-BB → brain, smooth muscle
Clinical Significance
Elevated CK-MB Activity
1. Acute Myocardial Infarction (Primary Use)
CK-MB activity kinetics:
- Rises: 3–6 hours after symptom onset
- Peaks: 12–24 hours
- Normalizes: 48–72 hours
Useful for:
- Reinfarction (second rise)
- Reperfusion monitoring
- Early MI detection
2. Myocardial Injury (Non-MI)
Elevated in:
- Myocarditis
- Pericarditis
- Tachyarrhythmias
- Cardiac surgery
- Cardioversion / defibrillation
- Ablation procedures
- Acute heart failure
3. Skeletal Muscle Disease (Minimal Elevation)
Because skeletal muscle contains small amounts of the MB isoform:
- Myositis
- Rhabdomyolysis
- Intense exercise
4. Renal Failure
Reduced clearance → mild elevation.
Low / Normal CK-MB
Normal CK-MB does not exclude MI (troponins more sensitive).
Useful as a baseline.
Reference Intervals
(Tietz 8E + IFCC + Mayo + ARUP)
Typical Adult Reference Range
- < 7 U/L
(Method-dependent; some labs use <5 U/L or <8 U/L)
Diagnostic Threshold for MI
- CK-MB activity > 2× upper limit of normal
or - CK-MB Index > 2.5–3%
CK-MB Index Formula
CK-MB Index=CK-MB (U/L)Total CK (U/L)×100\text{CK-MB Index} = \frac{\text{CK-MB (U/L)}}{\text{Total CK (U/L)}} \times 100CK-MB Index=Total CK (U/L)CK-MB (U/L)×100
> 2.5–3% → cardiac source
< 2.5% → skeletal muscle source
Summary Table
| Unit | Meaning |
| U/L or IU/L | µmol/min per liter |
| nkat/L | nmol/sec per liter |
| µkat/L | µmol/sec per liter |
| nmol/(s·L) | nanomole per second per liter |
| µmol/(s·L) | micromole per second per liter |
| µmol/(min·L) | micromole per minute per liter |
| µmol/(h·L) | micromole per hour per liter |
| µmol/(h·mL) | micromole per hour per milliliter |
Diagnostic Uses
1. Acute Myocardial Infarction (AMI)
- Early diagnostic marker
- Reinfarction detection
- Helps confirm myocardial injury when troponins borderline
- CK-MB index differentiates cardiac vs skeletal injury
2. Perioperative Cardiac Monitoring
Elevation after:
- CABG
- PCI
- Valve surgery
3. Cardiac Procedures
Monitor CK-MB after:
- Ablation
- Cardioversion
- Ventricular assist device procedures
4. Muscle-Related Disorders
Assess contribution of cardiac vs skeletal muscle in total CK elevation.
Analytical Notes
- Rate-kinetic IFCC standardized method preferred
- Hemolysis minimal effect
- Macro-CK variants may cause false elevation
- Serial measurements improve diagnostic accuracy
- CK-MB mass assay is more specific than activity assay
Clinical Pearls
- CK-MB activity remains valuable when troponin is chronically elevated (CKD, myocarditis).
- CK-MB peaks earlier and normalizes faster than troponin - helpful for reinfarction detection.
- High CK but low CK-MB index → skeletal muscle damage, not MI.
- CK-MB activity can be falsely high after cardiac procedures or defibrillation shocks.
- Always interpret with clinical symptoms + ECG findings.
Interesting Fact
Before the era of troponin, CK-MB activity was the gold standard for diagnosing MI and played a central role in emergency cardiac care for decades.
References
MedlinePlus / NIH - CK-MB Test.
Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Cardiac Enzymes.
IFCC Reference Methods for CK Isoenzymes.
ACC/AHA Guidelines for Acute MI.
Mayo Clinic Laboratories - CK-MB Activity.
ARUP Consult - Cardiac Biomarkers.