PHARMACOLOGY

 CLEARANCE ::

It is one of the most important concept of pharmacokinetics useful for designing drug dosage regimes Elimination of drug occurs mainly through kidney and hence renal -clearance is usually considered Renal clearance (CR) may be defined as the volume of plasma in ml which is cleared of a substance by kidney in one minute 

                    CR= UV

                            --------

                                P

where U= Concentration of drug in urine in mg/ml 

           V= Volume of urine (ml/min) excreted 

            P= Plasma drug concentration 

    Creatinine clearance is usually taken as the index of kidney function it is 97-140 ml/min in man and 85-125 ml/min in women 

BIOLOGICAL HALF-LIFE 

It is defined as the time required for the concentration of drug in the body to be reduced to one half (50%) of its steady -state 

Importance of Biological Half -life 

(1) It gives precise indication of disappearance or accumulation of the drug 

(2) It is used to estimate the time necessary to attain a new steady state level whenever a steady state is attained by a change in factors determining dosage regimes (e.g.drug dose -interval route of elimination volume of distribution etc ) 

(3) Half -life is an important tool to conclude whether a loading dose is necessary or not in multiple dosage regimen 

(4) Half life is indicative of diseased condition 

(5) It also shows the proportion of sound and unbound drugs 

            Half -life may be affected by following factors :;

(1) Renal insufficiency 

(2) Metabolic differences due to disease sex and age 

(3) Changes in urinary pH and 

(4) Co-administration of drug stimulation and inhibiting metabolism