What are the recommended doses of vancomycin and meropenem for a 3.9 kg baby with meningitis?

Antibiotic Dosing for Neonatal Meningitis in a 3.9 kg Infant

For a 3.9 kg baby with meningitis, administer vancomycin 60 mg (15 mg/kg) IV every 6 hours and meropenem 120 mg (30 mg/kg) IV every 8 hours.

Vancomycin Dosing

The recommended vancomycin dose for neonatal meningitis is 15 mg/kg/dose IV every 6 hours 1. For this 3.9 kg infant:

• Dose: 58.5 mg (round to 60 mg) IV every 6 hours
• Total daily dose: 240 mg/day (approximately 60 mg/kg/day) 1

Critical Vancomycin Considerations

• Vancomycin has poor CSF penetration (approximately 7% in non-inflamed meninges), which improves with meningeal inflammation 2, 3
• The CSF protein concentration/serum albumin ratio correlates strongly with vancomycin CSF penetration (r = 0.877, p < 0.005) 3
• Therapeutic drug monitoring (TDM) is essential - target serum trough concentrations of 15-20 mg/L to achieve adequate CSF levels 2
• In mild inflammatory states, vancomycin may not achieve adequate CSF concentrations even with appropriate serum levels 4

Meropenem Dosing

Based on the infant's gestational age and postnatal age, meropenem dosing varies 1, 5:

For infants ≥32 weeks gestational age:

• If postnatal age <14 days: 20 mg/kg/dose IV every 8 hours 1
• If postnatal age ≥14 days: 30 mg/kg/dose IV every 8 hours 1

For this 3.9 kg infant:

• If <14 days old: 78 mg (round to 80 mg) IV every 8 hours
• If ≥14 days old: 117 mg (round to 120 mg) IV every 8 hours

Meropenem Administration Details

• Administer as IV infusion over 15-30 minutes 5
• Meropenem achieves approximately 15% CSF penetration with inflamed meninges 2
• Meropenem is effective against H. influenzae, N. meningitidis, and penicillin-susceptible S. pneumoniae 1, 5
• For gram-negative meningitis or multidrug-resistant organisms, meropenem is preferred over imipenem due to lower seizure risk 1

Age-Specific Dosing Algorithm

Step 1: Determine gestational age (GA) and postnatal age (PNA)

Step 2: Apply appropriate dosing:

If GA <32 weeks:

• Vancomycin: 15 mg/kg IV every 6 hours 1
• Meropenem:
  • PNA <14 days: 20 mg/kg IV every 12 hours 1
  • PNA ≥14 days: 20 mg/kg IV every 8 hours 1

If GA ≥32 weeks (most likely for 3.9 kg infant):

• Vancomycin: 15 mg/kg IV every 6 hours 1
• Meropenem:
  • PNA <14 days: 20 mg/kg IV every 8 hours 1
  • PNA ≥14 days: 30 mg/kg IV every 8 hours 1

Critical Monitoring Requirements

• Obtain CSF cultures before initiating antibiotics 1
• Monitor vancomycin serum trough levels - draw before 4th dose, target 15-20 mg/L 2
• Assess renal function - adjust doses if creatinine clearance is impaired 5
• Monitor for seizures - particularly with meropenem, though risk is lower than imipenem 1
• Repeat lumbar puncture at 24-48 hours if clinical response is inadequate 1

Common Pitfalls to Avoid

• Do not underdose vancomycin - the 60 mg/kg/day total dose is necessary for meningitis, higher than the 40 mg/kg/day used for other infections 1
• Do not use ceftriaxone in neonates <28 days old due to risk of kernicterus from bilirubin displacement 1
• Do not delay antibiotics for lumbar puncture if the procedure cannot be performed immediately 1
• Do not assume adequate CSF penetration without considering meningeal inflammation severity - mild inflammation may result in subtherapeutic CSF levels despite adequate serum concentrations 4
• Do not use meropenem for highly cephalosporin-resistant pneumococcus - resistance patterns suggest limited efficacy 1

Combination Therapy Rationale

The combination of vancomycin plus meropenem provides:

• Gram-positive coverage (including MRSA and resistant S. pneumoniae) via vancomycin 1
• Gram-negative coverage (including H. influenzae, N. meningitidis, and enteric gram-negatives) via meropenem 1, 5
• Synergistic activity against certain pathogens 6
• Empiric coverage for both community-acquired and healthcare-associated pathogens 1