Whereas blood is collected for analysis at a discrete time point, urine is collected over periods of time known as collection intervals, for example 0-12 hours post dose, 12-24 hours post dose, and so on. The concentration of drug in urine is measured for each collection interval. If the total volume of urine in the collection interval is known, then the total amount of drug excreted during that collection interval can be calculated, with corrections made for specific gravity of urine, if required. These amounts can be summed to give the total amount of drug excreted in urine. This can be expressed as a fraction of total drug administered.
Typically, the amount of drug in each collection interval along with cumulative amounts of drug excreted are tabulated with appropriate descriptive statistics. Cumulative amounts may also be plotted against the mid-point of each collection interval.
Drug will often be metabolized and so the administered drug will be excreted as parent and / or metabolite(s). In this case the combined amounts of parent and metabolite (expressed as molar concentrations or corrected for molecular weight) will be tabulated.
Renal clearance is calculated as follows:
CLR = Ae / AUC
Where Ae is total amount of drug excreted in urine and AUC is the plasma AUC.
Metabolic clearance may be calculated by subtracting renal clearance from total clearance.
