Creatinine is the waste spontaneous product of creatine metabolism. It is an excellent marker of the renal function. The serum creatinine rate tends to remain constant. A high serum creatinine rate (associated to a high urea rate) corresponds to a decrease in renal glomerular filtration (FGR). The serum creatinine test is more reliable than the urea test. Indeed, the urea serum rate is affected by factors such as diet, dehydratation degree and protein metabolism (the serum creatinine rate is not influenced by these factors). The test of creatinine clearance can also be used to measure the FGR. In the case of renal transplantation, any increase in serum creatinine, as little as it may be, can reflect the rejection of the transplant. An increase of creatinine serum and urine can be the sign of muscular necrosis.