USMLE Step 1 Questions: Pharmacology Principles
USMLE Step 1 Practice Question: Pharmacology
To achieve a loading dose of a drug where the volume of distribution of the drug is 35 L and the clearance rate is 70 ml/min, and the desired dose is 3.5 mg/L, one should administer how many mg of drug initially?
(a) 70 mg
(b) 122 mg
(c) 175 mg
(d) 200 mg
(e) 225 mg
The answer is: B. The loading dose of a drug is defined as the [volume of distribution] X the [desired therapeutic plasma concentration]. The loading dose is independent of the clearance rate. Loading doses can be given intravenously and because the plasma concentration is reached almost immediately, the clearance rate can be ignored. If the volume of distribution is large, due for example to peripheral binding of the drug, then the volume of distribution is relatively high and the loading dose may be correspondingly high, requiring that the loading dose be administered more slowly to avoid toxicity. In the example in Item 48, the loading dose would be 35 L (volume of distribution) X 3.5 mg/L (therapeutic concentration) equal to 122.5 mg.
The maintenance dose of a drug is dependent upon the clearance rate. The dosing rate can be calculated by multiplying the clearance rate by the desired therapeutic plasma concentration. For this calculation, one does not need to know the volume of distribution for the drug.
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