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.

Creatinine is completely excreted in urine. It is filtered in the glomeruli and it is not secreted or reabsorbed in the renal tubules to any significant extent. So, its renal clearance reflects the glomerular filtration rate, which is a measure of the renal function.

The average normal creatinine clearance for a 70 kg individual is 120 mllmin. The decrease in kidney function results in lower creatinine clearance.

Creatinine clearance is an accurate and direct estimate of the renal function. It is more accurate that serum creatinine in determining the renal function.

The blood is filtered in the glomeruli at a rate of 120 ml/min. All small molecules with molecular weight below 2000 can be filtered freely in the glomeruli. Larger molecules can be filtered depending on their molecular size, however large protein molecules such as albumin cannot be filtered. So, drug molecules that are bound to plasma protein cannot be filtered and only free drug molecules can be filtered.

The rate of plasma filtration in the glomeruli is 120 ml/min and is called the glomerular filtration rate. The rate of drug filtration in the glomeruli is the product of the free (unbound) drug concentration in plasma and the GFR (i.e. fu Cp . GFR ).

Reduced kidney function leads to lower glomerular filtration rate. Also, some diseases (e.g. nephrotic syndrome) can affect the glomerular filtration process and allow the filtration of larger protein molecules such as albumin, and globulins.

1- Collect all the urine excreted by the patient over a period of 12 to 24 hr.

2- Obtain a plasma sample and determine creatinine plasma conc.

3- Determine the total volume of urine collected and creatinine conc in the urine.

4- Calculate the amount of creatinine excreted (Amount = volume X conc ).

5- Calculate the renal excretion rate.

6- Calculate the creatinine clearance