The therapeutic range is defined as the range of plasma drug concentrations in which the probability of the desired therapeutic effect is high and the probability of the unacceptable toxicity is low. The lower limit of the therapeutic range is the minimum effective concentration (MEC) and the upper limit of the therapeutic range is the minimum toxic concentration (MTC).

The therapeutic range represents a range of drug conc in plasma and not at the site of action. This means that when the plasma conc of the drug is within the therapeutic range, the drug conc at the site of action (may be lower or higher than the plasma conc) produces the desired therapeutic effect.

The therapeutic range is different for different drugs. When the therapeutic range is wide, the drug is said to have a large therapeutic index, while when the therapeutic range is narrow, it is said that the drug has small therapeutic index. Drugs with small therapeutic index require careful selection of the dosing regimen to achieve therapeutic drug concentration.

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.

It is the fraction of the administered iv dose which is excreted unchanged in urine. The amount of the drug excreted in urine is the total amount of the unchanged drug found in the urine when urine is collected from the time of drug administration until all the drug is eliminated from the body. If the drug is metabolized and then the metabolite is eliminated in urine, this is not included in the amount of drug excreted in urine.

The fraction ( f) is the ratio of the total amount excreted in urine to the iv administered dose. This fraction can range from 1 when all the drug is excreted in urine to 0 when the drug is not excreted in urine.

This fraction is determined from the ratio of:

- The renal elimination rate constant to the overall elimination rate constant

- The renal clearance to the total body clearance

The half life is the time required for the amount of the drug or the plasma concentration of the drug to decrease by 50%. The half life has units of time.

The half life and the first order elimination rate constant are dependent on the total body clearance and the volume of distribution.

The half life of a drug is constant within a patient (dose and concentration independent) when the drug elimination process follows first-order kinetics. However different patients may have different half lives for the same drug.

The half life is a very important parameter, because it indicates how fast the drug is eliminated from the body. Drugs with shorter half lives are eliminated faster than those with longer half lives.