Aminoglycoside drug levels, such as gentamicin, should be measured

• A. 30 minutes after a dose
• B. 3 hours after a dose
• C. At steady state
• D. At any time

Answer: (C)

Measuring aminoglycoside levels is most informative once the drug has reached steady state, because the amount eliminated between doses balances the amount given, creating stable, predictable concentrations. At steady state, trough levels (just before the next dose) reflect the drug’s accumulation and potential toxicity risk, while peak levels (taken at an appropriate time after a dose) reflect achievable bacterial killing. If levels are drawn before steady state—or too soon after a dose during distribution—the values can be misleading and lead to inappropriate dose adjustments. In practice, trough levels are commonly measured at steady state to guide safe dosing, with peaks assessed as needed to ensure efficacy.