Flucloxacillin Dosing Per Kilogram
For pediatric patients beyond the neonatal period, flucloxacillin should be dosed at 50 mg/kg every 6 hours IV for serious staphylococcal infections such as necrotizing fasciitis or bloodstream infections 1, 2.
Dosing by Age Group
Neonates and Premature Infants
Premature neonates (gestational age 26-29 weeks): 50 mg/kg IV, but dosing frequency must be adjusted based on age:
- First 2 weeks of life: every 12 hours (due to prolonged half-life of ~73 hours at birth, decreasing to ~46 hours by day 12) 3
- After 2 weeks: transition to more frequent dosing
Term neonates: Recent pharmacokinetic data suggests 25 mg/kg every 4 hours for all neonates regardless of gestational age when treating Staphylococcus aureus infections, as the current standard dosing (25-50 mg/kg every 8-12 hours) resulted in subtherapeutic levels in 31% of patients 4
Children Beyond Neonatal Period
- Standard dosing: 50 mg/kg/dose every 6 hours IV for serious infections 1, 2
- This is the recommended dose from IDSA guidelines for treating staphylococcal necrotizing infections
- Maximum single dose should not exceed adult dosing equivalents
Adults
- Standard dosing: 1-2 g every 4 hours IV 1, 2
- For severe infections: 2 g every 4 hours is preferred
- This translates to approximately 25-30 mg/kg every 4 hours for a 70 kg adult
Critical Considerations
Common pitfall: The evidence shows significant underdosing occurs with traditional neonatal regimens. The terminal half-life in neonates (4.6 hours) is much longer than adults, and clearance is substantially reduced (0.18 L/kg/h in neonates vs higher in adults) 3, 4. However, protein binding is also lower in neonates (86.3%) and even lower in critically ill patients (resulting in 11.5-16.7% unbound fraction vs ~5% in healthy adults) 5.
Temperature stability warning: If using outpatient parenteral therapy with elastomeric pumps, flucloxacillin solutions (50 mg/mL) maintain >92% potency for 24 hours at 33°C, but this drops to <85% at 37°C. Initial pH must be >5.9 to prevent precipitation 6.
Renal adjustment: Flucloxacillin clearance correlates with renal function. In neonates, renal clearance is only 0.182 mL/min/kg compared to higher values in adults 3. However, the correlation with gestational age and creatinine is low enough that fixed dosing by age is more practical than individual adjustment 4.
Target concentrations: Aim for time above MIC (T>MIC) >40% of the dosing interval. For S. aureus (MIC 2.0 mg/L), this requires the aggressive dosing outlined above. For coagulase-negative staphylococci (MIC 0.25 mg/L), doses can be reduced to 10 mg/kg every 6 hours 4.