Polymyxin B Dosing for a 2-Year-Old Child Weighing 30 lb (13.6 kg)
For a 2-year-old child weighing 13.6 kg, administer polymyxin B at 15,000–25,000 units/kg/day intravenously, divided every 12 hours, which translates to 204,000–340,000 units per day (102,000–170,000 units every 12 hours). Infants with normal kidney function may receive up to 40,000 units/kg/day (544,000 units/day for this child) without adverse effects 1.
Dosing Calculation and Administration
For this 13.6 kg child:
- Standard dose range: 15,000–25,000 units/kg/day = 204,000–340,000 units/day
- Divided dosing: Give every 12 hours (102,000–170,000 units per dose)
- Maximum safe dose for infants: Up to 40,000 units/kg/day = 544,000 units/day 1
Preparation: Dissolve 500,000 polymyxin B units in 300–500 mL of 5% Dextrose Injection for continuous intravenous drip 1.
Critical Dosing Considerations
Age-Specific Factors
- This child falls into the "infants and children" category per FDA labeling, where higher doses (up to 40,000 units/kg/day) have been used safely in limited studies, particularly for serious infections like Pseudomonas aeruginosa sepsis 1
- Doses as high as 45,000 units/kg/day have been studied in premature and newborn infants for P. aeruginosa sepsis 1
Renal Function Monitoring
- Baseline assessment required: Polymyxin B dosing must be reduced in renal impairment 1
- The standard dose assumes normal kidney function; any renal dysfunction necessitates downward adjustment from 15,000 units/kg 1
- Monitor renal function closely during therapy, as nephrotoxicity is dose-limiting 2, 3
Loading Dose Consideration
While the FDA label doesn't specify a loading dose for pediatric patients 1, adult guidelines recommend loading doses of 2–2.5 mg/kg to achieve rapid therapeutic levels 4. For severe infections in this child, consider consulting pediatric infectious disease specialists regarding loading dose appropriateness.
Safety and Monitoring
Avoid concomitant nephrotoxic agents: Do not combine with other nephrotoxic or ototoxic drugs, including vancomycin and aminoglycosides 2.
Maximum daily limit: The total daily dose must not exceed 25,000 units/kg/day for children with normal renal function (340,000 units/day for this 13.6 kg child) 1.
Therapeutic drug monitoring: If available, TDM should be performed to optimize efficacy and minimize toxicity 2. Target steady-state AUC of 50–100 mg·h/L is recommended in adults; pediatric targets are less well-defined.
Common Pitfalls
- Unit conversion errors: Polymyxin B sulfate uses 1 mg = 10,000 units. Pay careful attention to dosing units to avoid 10-fold errors 2
- Inadequate initial dosing: Starting at the lower end of the range (15,000 units/kg/day) may result in subtherapeutic exposure for serious infections; consider 20,000–25,000 units/kg/day for severe infections
- Failure to adjust for renal dysfunction: Even mild renal impairment requires dose reduction 1, 5
- Storage: Solutions must be refrigerated and discarded after 72 hours 1