(Apparent)volume of distribution The calculated volume of distribution (Vd)of a drug is an apparent volume that can be defined as the volume of fluid in which a drug would need to be dissolved to have the same concentration in that volume as it does in the plasma (B).The graph(C)provides an example of how the Vd is calculated.In this example,a dose of 500 mg was injected intravenously at time zero,and plasma drug concentrations were measured over time.The terminal elimination curve B was extrapolated back to time zero to determine that the plasma drug concentration at time zero,CO,was 5 mg/L.Then the Vd was calculated by dividing the dose by the Co.In this case,the result was 100 L.• The calculated volume of distribution (Vd) of a drug is an apparent volume that can be defined as the volume of fluid in which a drug would need to be dissolved to have the same concentration in that volume as it does in the plasma (B). The graph (C) provides an example of how the Vd is calculated. In this example, a dose of 500 mg was injected intravenously at time zero, and plasma drug concentrations were measured over time. The terminal elimination curve (β) was extrapolated back to time zero to determine that the plasma drug concentration at time zero, C0, was 5 mg/L. Then the Vd was calculated by dividing the dose by the C0. In this case, the result was 100 L. (Apparent) volume of distribution