Creatinine clearance is used to assess:

An estimate of creatinine clearance is based on the serum creatinine level and is used to measure GFR. Creatinine is a break-down product of creatine phosphate, which is found in muscle. It is freely filtered by the glomerulus, but also actively secreted by the peritubular capillaries in very small amounts such that creatinine clearance overestimates actual GFR by 10-20%. This margin of error is acceptable, considering the ease with which creatinine clearance is measured.