Creatine kinase (often abbreviated CK) is an enzyme that helps muscle cells and other cells with the need for rapid and constant use of energy to create chemicals that can store and release excess energy in the cells.
In most cells, adenosine triphosphate (ATP) is the chemical which provides its parts with usable energy. It is created when adenosine diphosphate reacts with oxygen and glucose and turns back into the diphosphate form when the energy is used. Creatine kinase plays a role in the creation and storage of ATP, making it easier to both create ATP and extract energy from it.
When muscle is damaged, it releases creatine kinase into the bloodstream. As such, creatine kinase is routinely tested for in blood tests for trauma victims, patients with chest pain, or patients with signs of kidney failure as excess levels in the blood can indicate a heart attack from a myocardial infarction, rhabdomyolysis, muscular dystrophy and acute kidney failure.
When Gregory House suffered his infarction, he showed elevated creatine kinase levels which were mistaken for muscle damage from drug injection, but were in fact from dying muscle tissue.