Which statement correctly defines loading dose and maintenance dose in PK dosing strategy?

• A. Loading dose rapidly achieves target concentration (LD ≈ Css × Vd), and maintenance dose maintains Css by MD = Css × Cl × τ.
• B. Loading dose is always smaller than maintenance dose.
• C. Loading dose is used after steady-state is reached.
• D. Maintenance dose rapidly achieves target concentration.

Answer: (A)

The idea being tested is how a loading dose and a maintenance dose work together to reach and hold a target plasma concentration. A loading dose is chosen to bring the concentration up to the desired steady-state level quickly, using the relationship LD = Css × Vd, where Css is the target steady-state concentration and Vd is the apparent volume of distribution. By delivering an amount proportional to how the drug distributes in the body, you jump to the target concentration rather than waiting for repeated doses to accumulate.

After reaching Css, the maintenance dose is used to keep the level there. That dose per dosing interval satisfies MD × (1/τ) = Css × (1/τ) × Cl, which simplifies to MD = Css × Cl × τ. Here Cl is clearance and τ is the dosing interval. This ensures the rate of input matches the rate of elimination, keeping the concentration steady at Css over time.

So the statement that correctly defines loading and maintenance dosing reflects that the loading dose rapidly achieves the target concentration using LD = Css × Vd, while the maintenance dose maintains Css by MD = Css × Cl × τ. The other ideas—loading dose being smaller than maintenance, loading dose used after steady-state is reached, or maintenance dose rapidly achieving the target—don’t align with how loading and maintenance dosing are designed to function.