Dosing Recommendations for Imipenem and Metronidazole in an 11-Year-Old, 50kg Patient
For this 11-year-old patient weighing 50kg, imipenem should be dosed at 1000-1250 mg (20-25 mg/kg) every 6 hours intravenously, and metronidazole should be dosed at 1500 mg (30 mg/kg) once daily intravenously or 500 mg every 8 hours.
Imipenem Dosing
Standard Pediatric Dosing
- The British Thoracic Society guidelines recommend 20-40 mg/kg (maximum 2g) three times daily intravenously for children 1.
- For a 50kg patient, this translates to 1000-2000 mg every 8 hours 1.
Weight-Based Considerations
- Since this patient weighs exactly 50kg, they fall into a transitional category where both pediatric and adult dosing may apply 1.
- The Infectious Diseases Society of America guidelines specify that adults >50kg should receive 1g twice daily, while adults <50kg should receive 15 mg/kg twice daily 1.
- However, for serious infections including complicated intra-abdominal infections, doses of 750 mg three times daily have been used in adults 1.
Optimized Dosing Based on Pharmacokinetics
- Recent population pharmacokinetic studies in children demonstrate that 25 mg/kg every 6 hours (100 mg/kg/day) via 1.5-hour infusions is required for adequate coverage against Pseudomonas aeruginosa 2.
- For this 50kg patient, this would be 1250 mg every 6 hours 2.
- Standard dosing of 15-25 mg/kg every 6 hours (750-1250 mg every 6 hours) is sufficient against most common bacterial isolates 1, 2.
Practical Recommendation
Dose imipenem at 1000 mg every 6 hours for most infections, or 1250 mg every 6 hours for suspected Pseudomonas or resistant organisms 1, 2.
Metronidazole Dosing
Standard Pediatric Dosing
- The IDSA guidelines recommend 30-40 mg/kg/day divided every 8 hours for pediatric patients 1.
- For a 50kg patient, this translates to 500 mg every 8 hours (1500 mg/day total) 1.
Once-Daily Dosing Option
- Recent pharmacokinetic-pharmacodynamic studies in pediatric appendicitis patients demonstrate that 30 mg/kg once daily (1500 mg for this patient) achieves adequate target attainment against Bacteroides fragilis with MIC ≤2 mcg/mL 3.
- This once-daily dosing achieved similar AUC values to adults receiving standard dosing regimens 3.
Practical Recommendation
Either 1500 mg once daily OR 500 mg every 8 hours is appropriate 1, 3. The once-daily regimen offers convenience and similar efficacy for most anaerobic infections 3.
Important Clinical Considerations
Renal Function Monitoring
- Dose reduction may be necessary in renal failure for both medications 1.
- Routine toxicity monitoring (CBC, renal function, liver function) should be performed intermittently throughout treatment 1.
Imipenem-Specific Warnings
- Seizure risk increases with imipenem, particularly in patients with renal insufficiency or CNS disease 1, 4, 5.
- Avoid concomitant use with valproate as imipenem reduces valproate serum concentrations 1.
- Increased seizure risk with concurrent ganciclovir 1.