**2.6 Administration of drug products and sampling**

Administration of drug products or formulations to the subjects should be based on randomization. After the administration, blood samples are withdrawn from the subjects at fixed time intervals. Some time is taken to withdraw a sample from each subject, and the total time difference between first subject and the last subject may range from 10 to 20 minutes depending upon the number of subjects and technicians involved in the study. If the sampling schedule is not followed rigorously in the same sequential manner, significant differences can conceivably exist in the actual duration of the drug in the body and the stated sampling time given for each subject. This 10 to 20 min difference in sample withdrawal from each subject during the study would represent a substantial change in the drug concentrations observed in the blood if under these conditions treatments are administered to the subjects in a sequential manner [20, 21].

If the bioavailability of a given dosage form is to be evaluated by a blood level study, some estimate of the area under the serum concentration versus time curve, peak plasma concentration (Cmax), and time of peak plasma concentration (Tmax) must be obtained from the study. Therefore, the frequency of sampling and the duration of sampling are very important for study. It will vary with the drug. There must be sufficient sampling points to allow for proper evaluation of the area under the blood level curve. A blood sampling done up to three to five half-lives of the drug, and if the half-life of the drug is not known, blood sampling should proceed until 1/10 or 1/20 of the peak levels are reached.

Urinary excretion studies are used when it is either not possible to measure a given drug in the blood, plasma or serum or when ethical considerations do not allow the collection of samples over a period of time. The advantage of this method is it involves noninvasive method of sampling, concentration of the drug in the urine is often greater than serum and the amount of the drug excreted in urine is obtained directly. But it is not useful in estimating the absorption rate of rapidly absorbed drugs and sometimes metabolites may interfere with the estimation of the unchanged drug in the urine sample.

Sampling must be continued for a sufficient time period to ensure that the area extrapolated from the time of the last measured concentration to infinite time should be less than 20% of the total AUC. AUC calculations are not useful in case of enterohepatic recycling where the terminal elimination rate constant cannot be calculated accurately. In such case, at least three sampling points from absorption phase, three to four points from Tmax and four points during the elimination phase has been taken. Intervals between successive sampling points in terminal elimination phase are used to calculate the elimination rate constant. It should not be longer than the half-life of the study drug.
