Creatinine is a waste product produced by muscle metabolism. Kidneys filter creatinine to be excreted through the urine and the renal function is reflected. Elevated levels indicate renal impairment. The opposite happens for urinary creatinine clearance; during renal impairment, urinary creatinine is extremely low. Measuring urinary creatinine clearance during unilateral kidney impairment is insignificant as the healthy kidney can still excrete creatinine through the urine.