The creatine kinase (CK) activity assay is a vital diagnostic tool used to measure the enzymatic activity of creatine kinase, an enzyme primarily found in the heart, brain, and skeletal muscles. This assay is crucial for evaluating muscle damage, as elevated CK levels are often indicative of conditions such as myocardial infarction, rhabdomyolysis, and other muscle-related disorders.

The assay typically employs a colorimetric method where creatine phosphate and adenosine diphosphate (ADP) are converted to creatine and adenosine triphosphate (ATP) by CK. The generated ATP is then utilized in a coupled reaction with hexokinase to phosphorylate glucose, producing glucose-6-phosphate, which is subsequently oxidized by glucose-6-phosphate dehydrogenase.

This process generates NADPH, measurable at 340 nm, with its intensity directly proportional to CK activity in the sample. The assay is versatile, applicable to various sample types including serum and plasma, making it suitable for both clinical diagnostics and research applications. With its high sensitivity and ability to be automated for high-throughput analysis, the CK activity assay plays a significant role in monitoring muscle health and assessing the effects of therapeutic interventions in metabolic diseases.