What is the appropriate meropenem dose for a 2.5‑month‑old infant weighing 2.6 kg?

Meropenem Dosing for 2.5-Month-Old Infant Weighing 2.6 kg

For this 2.5-month-old infant weighing 2.6 kg, administer meropenem 20 mg/kg every 12 hours (52 mg per dose every 12 hours), given as an intravenous infusion over 30 minutes. 1, 2

Dosing Algorithm Based on Age and Weight

This infant falls into a specific neonatal dosing category that requires careful consideration of both gestational age (GA) and postnatal age (PNA):

Step 1: Determine Gestational and Postnatal Age Category

• If gestational age <32 weeks AND postnatal age <14 days: 20 mg/kg every 12 hours 1
• If gestational age <32 weeks AND postnatal age ≥14 days: 20 mg/kg every 8 hours 1
• If gestational age ≥32 weeks AND postnatal age <14 days: 20 mg/kg every 8 hours 1
• If gestational age ≥32 weeks AND postnatal age ≥14 days: 30 mg/kg every 8 hours 1

Step 2: Calculate Actual Dose

For this 2.6 kg infant at 2.5 months (approximately 10 weeks) postnatal age:

• Most likely scenario (term infant): 30 mg/kg every 8 hours = 78 mg per dose every 8 hours 1
• If premature (<32 weeks GA): 20 mg/kg every 8 hours = 52 mg per dose every 8 hours 1

Step 3: Administration Guidelines

• Administer as intravenous infusion over 30 minutes 2
• Do not administer as bolus injection in infants less than 3 months of age 2
• Reconstitute with Sterile Water for Injection to achieve appropriate concentration 2

Critical Considerations for This Patient

Weight-Based Concerns

This infant's weight of 2.6 kg at 2.5 months of age is significantly below expected norms and suggests:

• Prematurity is highly likely - most term infants weigh 4-6 kg at this age 3
• Renal function assessment is essential - serum creatinine should guide dosing adjustments 3, 4
• Fluid overload status should be evaluated - though research suggests it may not significantly affect target attainment 5

Pharmacokinetic Optimization

The dosing must account for postmenstrual age (PMA) and renal function: 3, 4

• Meropenem clearance increases with postmenstrual age: CL (L/h/kg) = 0.041 + 0.040/SCr + 0.003 × (PCA-35) 4
• For adequate bacterial killing, maintain free drug concentrations >MIC for 40-75% of the dosing interval 3, 4
• The 20-30 mg/kg every 8-12 hour regimen achieves therapeutic targets in >90% of neonates 3, 4

Common Pitfalls to Avoid

• Do not use the standard pediatric dose of 10 mg/kg every 8 hours - this is only for children ≥3 months with complicated skin/soft tissue infections 2
• Do not assume normal renal function - obtain serum creatinine before dosing, as clearance is strongly associated with renal function 3, 4
• Do not administer as bolus injection - infants <3 months require 30-minute infusions 2
• Do not use adult dosing adjustments - neonatal pharmacokinetics differ substantially from older children and adults 3, 4

Indication-Specific Adjustments

If treating meningitis (not specified in question):

• Dose would be 40 mg/kg every 8 hours for children ≥3 months 2
• However, there is no established meningitis dosing for infants <3 months in the FDA label 2
• Meropenem achieves 70% CSF penetration in infants 3

If treating complicated intra-abdominal infection:

• Use the age-based dosing algorithm above (20-30 mg/kg every 8-12 hours) 1, 2, 3

Monitoring Requirements

• Obtain baseline serum creatinine before initiating therapy 3, 4
• Monitor renal function during treatment, especially if therapy extends beyond 48-72 hours 4
• Assess clinical response within 24-48 hours and adjust based on culture results 6
• Consider therapeutic drug monitoring in critically ill infants or those with suspected resistant organisms 6, 7