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.

It is the volume of the plasma or the blood that is completely cleared from the drug per unit time by the kidney. It has units of volume/time.

The renal clearance cannot exceed the CL_T. However it can be equal to the CL_T when the drug is excreted completely unchanged by the kidney.

The renal clearance can be determined from the renal excretion rate over a short interval and the average plasma concentration during this interval.

Bioavallability is a measure of the fraction of the administered dose that reaches the systemic circulation.

Absolute bioavailability is the fraction of the total dose that reaches the systemic circulation. It can have values between zero when the drug is not absorbed at all to 1 when all the administered dose reaches the systemic circulation. The absolute bioavailability can be expressed in terms of percentage (0% - 100%).

The relative bioavailability is the bioavailability of a drug product relative to the bioavailability of another drug product (reference standard preparation). The relative bioavailability can have any value above zero. It can be more than one when the product under investigation has bioavailability higher than that of the reference standard.

The volume of distribution is the apparent volume in which the drug can be distributed in. It is the factor that relates the amount of drug in the body to the concentration of the drug in the sampling site. It is not an actual volume, however it is a hypothetical volume determined by the drug distribution behavior.

The volume of distribution ranges from 3-5 liters to more than 25 L/kg. Drugs with higher affinity for tissues, have higher volume of distribution.