Clearance Rate as an Expression of Drug Elimination Rate
When a volume of blood or plasma is completely cleared of a drug over a unit time, it is called a clearance rate. It is useful in describing drug elimination from the body because it is related to blood or plasma perfusion of various organs of elimination, and it can be directly related to the physiological function of these organs. An example would be the renal clearance rate (RCR) of a drug.
Table 1: Drug Excreted into the Urine Versus Time
Time interval (hr)
Amount excreted (mg)
Cumulative amount excreted, DUa (mg)
DU(infinity)-DU (mg)
0.0-0.5
30.0
30.0
30.5
0.5-1.0
15.1
45.1
15.4
1.0-1.5
8.7
53.8
6.7
1.5-2.0
4.4
58.2
2.3
2.0-2.5
1.3
59.5
1.0
2.5-3.0
0.9
60.4
0.1
3.0-6.0
0.1
60.5
0.0
6.0-12.0
0.0
60.5
0.0
EXAMPLE 1: From the data gathered, the amount of drug excreted over the 0.0 - 0.5-hr interval was 30.0mg. If the plasma concentration at 0.25 hr was 8µg/ml, what was the renal clearance rate?
The result for this calculation is significant because the glomerular filtration rate (GFR) in normal males is approximately 125 ml/min and so suggests that the drug was eliminated by GFR. Had the RCR been less than 125ml/min, tubular reabsorption of the drug might have occurred. Had the result been greater than 125ml/min, tubular secretion may have been responsible in the elimination of the drug.
B.7c-2
