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The bioavailability of the drug being studied can be calculated by measuring the area under the curve (AUC) of the PO compared with the IV curves. The closest approximation of the area of the blue curve compared to the red curve is 1. Bioavailability is defined as the fraction of an administered drug dose that reaches systemic circulation. By definition, IV administration of a drug leads to a bioavailability of 1 against which all other delivery routes are compared. PO drug intake generally leads to decreased bioavailability compared with the IV formulation because of incomplete absorption, first-pass metabolism by the liver, and any distribution into other tissues that occurs before the drug enters systemic circulation. Calculation of bioavailability (F) = (oral AUC / oral dose) / (IV AUC * IV dose). Figure A shows the concentration over time curve for both the PO and IV formulations of a drug. Visually, the AUC of the blue curve and the red curve are approximately the same, meaning that the bioavailability would be close to 1. Incorrect Answers: Answer 1: 0.5 is the ratio of the peak concentration of PO vs IV medication (50/100 concentration). The bioavailability is instead defined as the ratio of the area under the curves of the IV vs PO routes, normalized to dosage. Answer 3: 2 is the ratio of the peak concentration of IV vs PO medication (100/50). While the peak concentrations can be useful in determining toxicity, the bioavailability describes the total dosage of medication that is delivered, which is better summarized by the area under the curve. Answer 4: 50 is the peak concentration of PO medication. The bioavailability describes the quantity of total medication that reaches systemic circulation rather than the peak concentration. The peak concentration can be adjusted by changing the oral formulation (such as immediate vs delayed release). Answer 5: 100 is the peak concentration of IV medication. By definition, the bioavailability of IV medication is set at 1 and the ratio of any other delivery method is compared to this value. Bullet Summary: Bioavailability is defined as the fraction of an administered drug dose that reaches systemic circulation.
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