Intramuscular Administration of Ampicillin and Gentamicin in Neonates
For neonates requiring intramuscular antibiotics, administer ampicillin 150 mg/kg/day divided every 8 hours (50 mg/kg/dose) and gentamicin 4 mg/kg once daily for infants 8-21 days old with suspected serious infection. 1
Age-Stratified Dosing Algorithm
For Infants 8-21 Days Old (Most Common Scenario)
Ampicillin Dosing:
- UTI or no identified focus: 150 mg/kg/day IM divided every 8 hours (50 mg/kg per dose) 1
- Bacterial meningitis: 300 mg/kg/day IM divided every 6 hours (50 mg/kg per dose) 1
Gentamicin Dosing:
- Standard dose: 4 mg/kg IM once every 24 hours 1
- This once-daily dosing provides synergy with ampicillin against Group B Streptococcus and enterococcal species 1
For Infants 22-28 Days Old
- Transition to ceftriaxone 50 mg/kg IM once daily is preferred over ampicillin-gentamicin combination 1
- If ampicillin-gentamicin is used, follow the 8-21 day dosing above 1
For Neonates ≤7 Days Old (If Applicable)
Weight-based ampicillin dosing from FDA label:
- Birth weight ≤2000g: 50 mg/kg/day divided every 12 hours 2
- Birth weight >2000g: 75 mg/kg/day divided every 8 hours 2
- For meningitis: 100 mg/kg/day divided every 12 hours regardless of weight 2
Gentamicin for neonates ≤7 days:
Practical Administration Technique
Reconstitution for IM Injection
Ampicillin preparation:
- For 250 mg vial: Add 1 mL Sterile Water for Injection to achieve 250 mg/mL concentration 2
- For 500 mg vial: Add 1.8 mL Sterile Water for Injection to achieve 250 mg/mL concentration 2
- For 1 gram vial: Add 3.5 mL Sterile Water for Injection to achieve 250 mg/mL concentration 2
- Use only freshly prepared solutions within one hour as potency decreases significantly after this period 2
Gentamicin preparation:
- Available as 40 mg/mL solution requiring no reconstitution 3
- Calculate volume based on 4 mg/kg dose for infants 8-21 days old 1
Injection Sites and Technique
- Administer deep IM into the anterolateral thigh (vastus lateralis muscle) - the preferred site in neonates
- Rotate injection sites with each dose to minimize local tissue irritation
- Use separate syringes and injection sites for ampicillin and gentamicin - never mix these antibiotics in the same syringe 2
Critical Monitoring Parameters
Serum Gentamicin Levels
- Peak concentration (30-60 minutes post-IM): Target 4-6 mcg/mL 3
- Trough concentration (just before next dose): Must be <2 mcg/mL, ideally <1 mcg/mL 3, 5
- Measure both peak and trough levels periodically during therapy to ensure adequate but not excessive drug levels 3
- The once-daily 4 mg/kg dosing achieves therapeutic peaks in 97% of neonates while maintaining safe trough levels in 93% 5
Renal Function Monitoring
- Measure serum creatinine on days 1 and 3 of therapy 5
- If creatinine clearance <50 mL/min, consult infectious disease specialist for dose adjustment 3
Clinical Response Assessment
- Reassess within 48-72 hours if no clinical improvement is evident 6, 7
- Continue treatment for minimum 48-72 hours beyond resolution of symptoms 2
- Minimum 10 days total treatment for Group A beta-hemolytic streptococcal infections 2
Common Pitfalls to Avoid
Do not assume standard pediatric dosing applies to neonates - they require age-specific and weight-specific adjustments due to immature renal function and altered pharmacokinetics 6, 3
Avoid the traditional twice-daily gentamicin dosing (2.5 mg/kg every 12 hours) in infants >7 days old, as this frequently results in inadequate serum levels with 22-53% of neonates having subtherapeutic trough concentrations 4, 5
Never administer ampicillin more rapidly than recommended - rapid IV administration can cause convulsive seizures, though this is less relevant for IM dosing 2
Do not use solutions prepared more than one hour prior for ampicillin, as potency decreases significantly 2
Recognize that complications from IM administration include infiltration, infection, and nerve compression (particularly in ankle injections), though these are rare 1