Ampicillin and Gentamicin Dosing for a 920g Preterm Neonate
For a 920g preterm infant, administer ampicillin 50 mg/kg/day divided every 12 hours (25 mg/kg/dose IV q12h) and gentamicin 3.5 mg/kg IV every 24 hours.
Ampicillin Dosing
The dosing for this extremely low birth weight infant depends critically on postnatal age:
If ≤7 Days Postnatal Age:
- 50 mg/kg/day divided every 12 hours (25 mg/kg/dose IV q12h) 1
- This applies to neonates ≤2000g during the first week of life 1
If >7 Days Postnatal Age:
- 50 mg/kg/day divided every 12 hours (25 mg/kg/dose IV q12h) 1
- For infants <1200g beyond the first week, this lower dose frequency is maintained due to immature renal clearance 1
Key Point: At 920g, this infant falls into the <1200g weight category, which requires the most conservative dosing regardless of postnatal age 1. The 2022 guidelines confirm gestational age <34 weeks with weight considerations: for infants >34 weeks gestation but <1200g, the same 50 mg/kg/day q12h regimen applies 1.
Gentamicin Dosing
For premature neonates <1000g: 3.5 mg/kg IV every 24 hours 1
This once-daily extended-interval dosing is specifically designed for extremely premature infants with:
- Larger volume of distribution
- Reduced renal clearance
- Need for concentration-dependent bacterial killing 2
Alternative Dosing from 2022 Guidelines:
The more recent guidelines suggest for gestational age <30 weeks 1:
- If postnatal age <14 days: 5 mg/kg/dose IV every 48 hours
- If postnatal age >14 days: 5 mg/kg/dose IV every 36 hours
However, the 3.5 mg/kg every 24 hours regimen from the 2009 IDSA guidelines remains the most widely validated approach for infants <1000g 1, with extensive safety data showing 100% achievement of non-toxic trough levels 2.
Critical Monitoring Requirements
Gentamicin Levels:
- Obtain trough level before the 3rd or 4th dose (target <2 mcg/mL) 2
- Obtain peak level 30 minutes after infusion (target 6-12 mcg/mL) 2
- Adjust dosing interval if trough >2 mcg/mL to prevent nephrotoxicity and ototoxicity 1, 2
Renal Function:
- Monitor serum creatinine on days 1 and 3 of therapy 3
- Assess for signs of nephrotoxicity throughout treatment 3
Ototoxicity Screening:
- Perform hearing assessment if prolonged therapy (>72 hours) 3
Common Pitfalls to Avoid
Do not use weight-based dosing intended for larger neonates - the 2.5 mg/kg q12h or q18-24h regimens are for infants 1200-2000g, not <1000g 1
Do not use higher ampicillin doses (75-100 mg/kg/day) - these are reserved for larger or older neonates and risk toxicity in extremely low birth weight infants 1
Avoid checking levels too early - gentamicin requires at least 48-72 hours to reach steady state in premature neonates 2
Do not assume gestational age alone determines dosing - actual birth weight is equally critical, particularly for small-for-gestational-age infants who may have lower drug levels 4
Practical Administration
- Ampicillin: For a 920g infant, each dose = 23 mg IV q12h (50 mg/kg/day ÷ 2 doses)
- Gentamicin: For a 920g infant, dose = 3.2 mg IV q24h (3.5 mg/kg × 0.92 kg)
Both medications should be administered as slow IV infusions over 30 minutes to minimize adverse effects 1.