Polymyxin B Conversion to International Units
For polymyxin B sulfate, the conversion is 1 mg = 10,000 IU 1, 2.
Conversion Factor
The FDA label explicitly states that each milligram of pure polymyxin B base is equivalent to 10,000 units of polymyxin B 2. This conversion is consistently referenced in clinical guidelines for carbapenem-resistant gram-negative bacterial infections 1.
Dosing for a 70-kg, 80-year-old Male
Loading Dose
Administer 200-300 mg (2,000-3,000 IU) as a loading dose 3, 4.
- For this specific patient: 250 mg = 2,500,000 IU would be appropriate
- Loading doses ensure rapid achievement of therapeutic concentrations regardless of renal function
Maintenance Dose
Administer 150 mg (1,500,000 IU) every 12 hours 5, 4.
- Recent pharmacokinetic data in elderly patients (≥65 years) demonstrates that fixed dosing of 75 mg twice daily (total 150 mg/day) achieves optimal exposure while minimizing nephrotoxicity risk 5
- This translates to 750,000 IU every 12 hours
- For this 70-kg patient, this represents approximately 2.1 mg/kg/day (21,000 IU/kg/day)
Critical Considerations for Elderly Patients
Age-related factors significantly impact polymyxin B pharmacokinetics in this 80-year-old patient:
- Elderly patients show altered clearance patterns, with albumin being a significant covariate affecting drug exposure 5
- Fixed maintenance dosing (rather than weight-based) maximizes efficacy while balancing nephrotoxicity concerns in elderly patients 5
- Weight-based regimens of 1.25-1.5 mg/kg twice daily in elderly patients may result in >40% experiencing toxic exposures (AUCss,24h >100 mg·h/L) 5
Monitoring Requirements
Implement therapeutic drug monitoring (TDM) and renal function surveillance:
- Target AUC24h of 50-100 mg·h/L for optimal efficacy with acceptable nephrotoxicity 5, 4
- Monitor serum creatinine closely, as 58% of patients develop acute kidney injury with high-dose polymyxin B 6
- Avoid concurrent nephrotoxins, which independently increase AKI risk (aOR 2.14) 6
Renal Function Adjustments
If this patient has renal insufficiency (common in elderly):
- No dose reduction needed for maintenance dosing unless on continuous renal replacement therapy 3
- Patients with GFR <80 mL/min show decreased clearance (2.1 L/h vs 3.9 L/h), increasing exposure risk 3
- For patients on CVVHD: maintain 100 mg (1,000 IU) every 12 hours after loading dose 3
Common Pitfall
Avoid weight-based dosing in elderly patients - the evidence strongly supports fixed dosing regimens in this population to prevent supratherapeutic exposures and associated nephrotoxicity 5. The substantial pharmacokinetic variability observed across patients (clearance range 2-3.9 L/h) reinforces the need for TDM rather than relying solely on weight-based calculations 3, 4.