CK and CPK Are the Same Enzyme
Creatine Kinase (CK) and Creatine Phosphokinase (CPK) are identical—they are simply two different names for the exact same enzyme. 1 The terms are used interchangeably in clinical practice, with "CK" being the more commonly used abbreviation in contemporary medical literature.
Understanding the Nomenclature
- CK is the preferred modern terminology used in current guidelines and clinical practice, though CPK remains in use particularly in older literature and some clinical settings 2
- Both terms refer to the enzyme that catalyzes the reversible reaction of phosphocreatine, magnesium adenosine diphosphate (MgADP-), and a hydrogen ion to MgATP2- and creatine 1
- The enzyme plays a crucial role in cellular energy metabolism by either re-phosphorylating ADP to ATP or storing immediately available energy in the form of phosphocreatine 1
Clinical Significance and Isoenzymes
The enzyme exists in three main cytoplasmic isoenzymes that provide tissue-specific diagnostic information:
- CK-MM (or CPK-MM): Predominantly found in skeletal muscle, useful for diagnosing skeletal muscle diseases such as muscular dystrophy 3, 4
- CK-MB (or CPK-MB): Found in myocardium, historically the standard marker for myocardial infarction diagnosis and remains useful for detecting early reinfarction due to its shorter half-life compared to troponins 2, 3
- CK-BB (or CPK-BB): Found in brain tissue, useful in brain damage and certain malignancies of the gastrointestinal tract 3, 4
Diagnostic Applications
- When CK-MB is measured, mass immunoassays should be used rather than older methods for optimal accuracy 2
- Elevated total CK without elevated CK-MB suggests skeletal muscle injury rather than cardiac injury 1
- CK-MB has been largely superseded by cardiac troponins (troponin T and I) as the preferred markers for myocardial necrosis due to troponins' greater sensitivity and specificity 2
- CK-MB remains clinically valuable in specific scenarios: diagnosing early infarct extension/reinfarction and evaluating periprocedural myocardial injury during cardiac interventions 2, 1
Important Clinical Considerations
- The enzyme has a molecular size of approximately 82 kDa, which prevents it from entering the bloodstream under normal conditions 1, 5
- CK levels commonly increase after unaccustomed exercise, especially with eccentric contractions, which can complicate interpretation when evaluating for cardiac or muscle pathology 1, 6
- Physical activity significantly affects CK variability, emphasizing the need for caution when using CK measurements to diagnose acute myocardial infarction or skeletal muscle injury 6
- Nonmodifiable factors including ethnicity, age, and gender can affect enzyme tissue activity and subsequent serum CK levels 7