In a 6‑year‑old child receiving gentamicin, the trough concentration drawn 8 hours after the dose is <0.5 µg/mL and no peak level was obtained; is this trough acceptable and what steps should be taken to assess therapeutic efficacy and adjust dosing?

Gentamicin Pharmacokinetics in a 6-Year-Old: Trough Assessment

A trough of <0.5 µg/mL drawn 8 hours post-dose is NOT a true trough and cannot be used to assess therapeutic adequacy or guide dosing adjustments; you must obtain a proper pre-dose trough concentration and a peak level to ensure both efficacy and safety.

Critical Problem with This Scenario

The fundamental issue is timing of the sample:

• True trough definition: A trough must be drawn immediately before the next scheduled dose, not 8 hours after administration 1
• 8-hour post-dose level: This represents a mid-interval concentration, not a trough, and provides limited pharmacokinetic information 2
• Gentamicin half-life in children: Typically 2-3 hours in patients with normal renal function, meaning an 8-hour level is approximately 2-3 half-lives post-dose 3, 4

What This Level Actually Tells You

The <0.5 µg/mL concentration at 8 hours post-dose suggests:

• Rapid clearance: The drug is being eliminated quickly, which could indicate normal or enhanced renal function 3
• Possible underdosing: If the level is already this low at 8 hours, the peak may have been subtherapeutic 1, 4
• Cannot assess nephrotoxicity risk: Without a true pre-dose trough, you cannot determine if drug accumulation is occurring 5, 3

Required Actions for Proper Therapeutic Drug Monitoring

Immediate Steps

Obtain proper pharmacokinetic samples 1, 5:

• Peak concentration: Draw 30-60 minutes after completion of IV infusion (or 1 hour after IM injection)
  • Target: 3-4 µg/mL for synergy dosing 1
  • Target: 5-10 µg/mL for Gram-negative coverage 1
• True trough concentration: Draw immediately before the next scheduled dose
  • Target: <1 µg/mL (preferably <0.5 µg/mL) 1, 5, 3

Dosing Considerations for Pediatric Patients

Standard pediatric dosing (children >1 month with normal renal function) 1:

• For synergy (e.g., endocarditis): 3-6 mg/kg/day divided every 8 hours
• For Gram-negative infections: 7.5 mg/kg/day divided every 8 hours

Once-daily dosing alternative 4:

• 7.5 mg/kg every 24 hours has been validated in pediatric patients aged 6 months to 18 years
• Achieves higher peaks (mean 20.4 µg/mL) and lower troughs (mean 0.29 µg/mL) compared to traditional dosing
• However, insufficient clinical experience for endocarditis treatment in children 1

Clinical Context Matters

If This Is for Endocarditis

Multiple divided doses are mandatory 1:

• Gentamicin must be given every 8 hours, not once daily
• Once-daily dosing has not been adequately studied for enterococcal endocarditis and is not recommended 1
• Peak and trough monitoring is essential to balance efficacy and nephrotoxicity risk 1

If This Is for Gram-Negative Infection

Once-daily dosing may be appropriate 3, 4:

• 7 mg/kg every 24 hours based on total body weight
• Higher probability of achieving PK/PD targets (peak:MIC ratio >8-10)
• Reduced nephrotoxicity risk with lower trough concentrations

Monitoring Protocol Going Forward

Mandatory monitoring for all patients receiving >1 dose 5, 3:

1. Baseline assessment: Serum creatinine, calculated creatinine clearance
2. Peak and trough after first dose: To confirm therapeutic targets
3. Trough monitoring: Before every 3rd-4th dose if therapy continues
4. Renal function: Serum creatinine at least twice weekly 5
5. Audiology assessment: If treatment exceeds 7-10 days 3

Common Pitfalls to Avoid

• Do not assume goal trough based on improperly timed sample: An 8-hour level cannot substitute for a true trough 1, 5
• Do not continue dosing without peak level: You cannot assess efficacy without knowing if therapeutic peaks are achieved 1, 3
• Do not use once-daily dosing for endocarditis: This is only validated for Gram-negative infections, not synergy-based regimens 1
• Do not ignore the clinical indication: Dosing targets differ significantly between synergy dosing (endocarditis) and monotherapy (UTI, pneumonia) 1, 3

Bottom Line

Redraw levels at appropriate times: Obtain a peak 30-60 minutes after the next dose and a true trough immediately before the subsequent dose to properly assess this patient's gentamicin pharmacokinetics and make evidence-based dosing adjustments 1, 5, 3.