This chapter discusses serum creatinine estimation and the creatinine clearance test, where serum creatinine level can be used as an indicator of renal function. Creatinine is formed from creatine phosphate which is the storage form of energy in the muscle. Creatinine clearance determination does not require administration of exogenous creatinine, as it is a product of muscle metabolism. Creatinine is completely filtered at the glomerulus of the kidney with very small amounts reabsorbed and secreted at the tubules. Decreased creatinine clearance is a very sensitive indicator of reduced glomerular filtration rate due to renal damage. The importance of creatinine clearance is in the early detection of impairment of renal function without obvious signs and symptoms. The importance of creatinine clearance is in the early detection of impairment of renal function without obvious signs and symptoms. The creatinine clearance test is also useful in monitoring of a patient with renal insufficiency.