In a 6‑year‑old child receiving gentamicin 2.5 mg/kg every 8 hours for gram‑positive synergy, with a trough drawn 6 hours after the previous dose that is <0.5 µg/mL and stable renal function, should the current dosing be continued or adjusted?

Gentamicin Dosing Assessment in a 6-Year-Old Receiving Synergy Therapy

The current dosing regimen should be continued without adjustment, but a true trough level must be obtained immediately before the next scheduled dose to properly assess nephrotoxicity risk. The level drawn 6 hours after infusion ended is a mid-interval concentration, not a true trough, and cannot be used to guide therapy. 1

Critical Issue: Improper Trough Timing

• A true trough concentration must be obtained immediately before the next scheduled dose; samples drawn at any other time (including 6 hours post-infusion) represent mid-interval levels and cannot substitute for therapeutic adequacy or safety assessment. 1
• The reported level of <0.5 µg/mL at 6 hours post-infusion does not confirm that the pre-dose trough will be in the safe range (<1 µg/mL, preferably <0.5 µg/mL). 1
• Do not treat an 8-hour post-dose level as a trough; it cannot assess nephrotoxicity risk and does not allow proper therapeutic monitoring. 1

Current Dosing Appropriateness for Gram-Positive Synergy

• The current regimen of 2.5 mg/kg every 8 hours is appropriate for gram-positive synergy (endocarditis) in pediatric patients, as guidelines recommend 3 mg/kg/day divided into three equal doses every 8 hours. 2
• Multiple divided doses (every 8 hours) are mandatory for gentamicin when used for synergy in endocarditis; once-daily dosing has not been adequately studied in this indication and is not recommended. 1, 3
• The dose of 2.5 mg/kg every 8 hours (7.5 mg/kg/day total) is within the recommended pediatric synergy dosing range of 3–6 mg/kg/day divided every 8 hours. 1

Required Monitoring Protocol

• Peak sampling should be performed 30–60 minutes after the end of the IV infusion, with target peaks of 3–4 µg/mL for synergy regimens (endocarditis). 1, 4
• True trough sampling must be drawn right before the next dose, with desired troughs <1 µg/mL (preferably <0.5 µg/mL) to minimize nephrotoxicity while maintaining efficacy. 1, 5, 6
• Do not continue dosing without a documented peak level, as therapeutic efficacy cannot be confirmed without verifying adequate drug exposure. 1
• Serum creatinine should be monitored at least twice weekly during therapy. 1

Common Pitfall to Avoid

• The most critical error in this case is accepting the 6-hour post-infusion level as a valid trough. This timing represents a mid-interval concentration that will be significantly higher than the true pre-dose trough, potentially masking drug accumulation and nephrotoxicity risk. 1
• The patient requires both a properly timed peak (30–60 minutes post-infusion) and a true trough (immediately pre-dose) to ensure the regimen is both effective and safe. 1, 4

Recommended Action Plan

• Continue the current dose of 2.5 mg/kg every 8 hours as it is appropriate for gram-positive synergy. 2
• Obtain a peak level 30–60 minutes after the next infusion to confirm therapeutic efficacy (target 3–4 µg/mL). 1, 4
• Obtain a true trough level immediately before a scheduled dose to assess nephrotoxicity risk (target <0.5–1 µg/mL). 1, 5
• Monitor renal function (serum creatinine) at least twice weekly throughout therapy. 1
• Consider audiology evaluation if treatment exceeds 7–10 days. 1