When should the next dose of the drug be given to maintain a serum level within the therapeutic range of 30-45 mg/dL, given a current serum level of 40 mg/dL and a half-life of 3 hours?

Dosing Interval Based on Half-Life and Therapeutic Range

The next dose should be given in approximately 1 hour (specifically between 0.5 to 1.5 hours) to maintain the serum level within the therapeutic range.

Pharmacokinetic Calculation

Using first-order elimination kinetics, with a current level of 40 mg/dL and a half-life of 3 hours, the serum concentration will decline to 30 mg/dL (the lower limit of the therapeutic range) in approximately 1 hour. 1

• The calculation follows the exponential decay formula: at 1 half-life (3 hours), the level drops to 20 mg/dL, which would be below the therapeutic range 1
• To maintain levels between 30-45 mg/dL, dosing must occur before the concentration falls below 30 mg/dL 2
• Since the drug decreases from 40 to 30 mg/dL in approximately 1 hour (25% reduction), this represents the optimal redosing interval 1

Clinical Rationale for Timing

Therapeutic drug monitoring principles dictate that redosing should occur when serum concentrations approach the lower limit of the therapeutic range to avoid subtherapeutic levels while preventing accumulation. 2

• Waiting longer than 1-1.5 hours risks dropping below 30 mg/dL, potentially compromising therapeutic efficacy 1, 2
• Dosing earlier than 1 hour maintains levels above 40 mg/dL, which remains safely within the 30-45 mg/dL therapeutic window 1
• The goal is to maintain steady-state concentrations within the narrow therapeutic range without excessive peak-trough fluctuations 2

Monitoring Considerations

Serum level measurements should be obtained as trough concentrations (immediately before the next scheduled dose) to guide ongoing dosage adjustments. 2

• Trough levels provide the most reliable assessment of whether therapeutic concentrations are maintained throughout the dosing interval 3, 2
• If trough levels consistently fall below 30 mg/dL with this dosing interval, either the dose amount should be increased or the interval shortened further 1, 2
• Peak levels should also be monitored to ensure they do not exceed 45 mg/dL, which would indicate excessive dosing 2