Different tests. Creatinine is a waste product of (mainly muscle) metabolism, & builds up when the body (kidney) cannot get rid of it. Creatine Phosphokinase, AKA Total CPK, is an enzyme produced by certain cells in the body that produces creatine byproducts (muscle, bone, brain, heart have CPK). An elevated CPK means one of these organs is/was damaged in some way. So the 2 tests measure different things.
Creatinine vs CK. Creatinine is a molecule released from muscle that circulates briefly and is excreted promptly by our kidneys. Creatinine is an energy holding and transfer molecule in muscle cells. It is measured routinely in blood tests as an indicator of kidney filtration efficiency. It is the standard marker of kidney function. Creatine kinase is a muscular enzyme which catalyzes reactions in cells. .