What is the effect on apparent Vd when red blood cells act as a reservoir for a drug due to binding?

What is the effect on apparent Vd when red blood cells act as a reservoir for a drug due to binding?

• A. Apparent Vd increases
• B. Apparent Vd decreases
• C. Apparent Vd remains the same
• D. Apparent Vd becomes negative

Answer: (A)

Apparent Vd reflects how widely a drug distributes beyond the plasma. When red blood cells bind the drug and act as a reservoir, a portion of the drug is held in the blood and is not freely circulating in plasma. For a given total dose, this lowers the plasma concentration, and since Vd is calculated as Dose divided by the plasma concentration, the ratio increases. In short, RBC binding pulls more drug into the blood compartment and away from plasma, making the apparent volume of distribution larger. This is why the apparent Vd rises rather than falls, stays the same, or becomes negative.

Apparent Vd reflects how widely a drug distributes beyond the plasma. When red blood cells bind the drug and act as a reservoir, a portion of the drug is held in the blood and is not freely circulating in plasma. For a given total dose, this lowers the plasma concentration, and since Vd is calculated as Dose divided by the plasma concentration, the ratio increases. In short, RBC binding pulls more drug into the blood compartment and away from plasma, making the apparent volume of distribution larger. This is why the apparent Vd rises rather than falls, stays the same, or becomes negative.