The fraction remaining of the normal kidney function. This is estimated by comparing the kidney function of the patient with the normal kidney function.

The creatinine clearance is a diagnostic test which is highly correlated with the kidney function. So, the creatinine clearance is used as a measure of the kidney function.

The kidney function can range from 1 when the patient's kidney function is normal to 0 when the patient's kidney function is completely impaired (anureic patients).

The creatinine clearance can be determined directly from urinary excretion rate and serum creatinine. Also, the creatinine clearance can be estimated from serum creatinine, age, and weight.

- Patients with lower kidney function ( lower KF ) have slower rate of renal drug elimination. The decrease in the rate of renal drug elimination will be reflected on the overall drug elimination rate.

- Patients with lower kidney function ( lower KF ) require larger reduction in their doses to achieve therapeutic drug concentrations.