Calculation of Loading and Maintenance Doses determines initial and sustaining doses needed to achieve and maintain therapeutic levels.
Loading Dose
Definition & Purpose
- A loading dose is an initial higher dose of a drug given to rapidly achieve the therapeutic concentration in the plasma.
- Useful for drugs with long half-lives, where waiting 4–5 half-lives for steady state might be clinically unacceptable (e.g., in urgent situations).
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Formula
$\text{Loading Dose} = V_d \times C_{\text{target}}$
- Vd = Volume of distribution
- Ctarget = Desired plasma drug concentration
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Key Insight
- The loading dose immediately brings the plasma concentration closer to the desired therapeutic level, while subsequent maintenance doses keep it within the target range.
Maintenance Dose
Definition & Purpose
- A maintenance dose is the dose required to replace the amount of drug lost per unit time (through metabolism and excretion) and maintain steady state.
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Formula (for IV infusion or repeated dosing):
$\text{Maintenance Dose Rate} = CL \times C_{\text{target}}$
- CL = Clearance
- Ctarget = Desired plasma concentration
When dosing at discrete intervals τ:
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$\text{Maintenance Dose} = (CL \times C_{\text{target}}) \times \tau$
Interpretation
- The maintenance dose rate is set so that the input (dose) balances the output (elimination) over time, maintaining steady state.
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