Polymyxin B Dosing for 70kg Patient
For a 70kg patient, administer a loading dose of 140-175 mg (2-2.5 mg/kg) followed by a maintenance dose of 105-210 mg/day (1.5-3 mg/kg/day) divided into two doses given every 12 hours, with no dose adjustment required regardless of renal function. 1, 2
Practical Dosing Calculation
Given a 5L units vial (500,000 units), this equals 50 mg of polymyxin B (since 1 mg = 10,000 units). 3
For your 70kg patient:
- Loading dose: 140-175 mg (approximately 3-3.5 vials of 500,000 units each) 1, 2
- Maintenance dose: 52.5-105 mg every 12 hours (approximately 1-2 vials per dose) 1, 2
Critical Dosing Principles
Polymyxin B clearance is independent of renal function and should NOT be dose-adjusted for renal impairment, unlike colistin which requires significant renal dose adjustment. 1, 4, 5 This represents a major departure from older FDA labeling that recommended renal dose adjustment. 2
The evidence strongly demonstrates:
- Total body clearance of polymyxin B shows no correlation with creatinine clearance (r² = 0.008), even in patients with creatinine clearance ranging from 10-143 mL/min. 5
- Polymyxin B is predominantly non-renally cleared with median urinary recovery of only 4.04%. 5
- Steady-state exposures are comparable between patients with normal renal function (AUC 63.5 ± 16.6 mg·h/L) versus renal insufficiency (AUC 56.0 ± 17.5 mg·h/L, P = 0.42). 4
Loading Dose Imperative
Always administer the full loading dose of 2-2.5 mg/kg (140-175 mg for 70kg) regardless of renal function to achieve therapeutic levels on day one. 1 This is critical because polymyxin B has a relatively long half-life and without a loading dose, therapeutic concentrations are delayed by days. 1
Maintenance Dosing Strategy
The recommended maintenance dose range is 1.5-3 mg/kg/day divided every 12 hours. 3, 1, 2 However, recent pharmacokinetic/pharmacodynamic data suggests:
- For Pseudomonas aeruginosa with MIC ≤1 mg/L: 3.5 mg/kg/day achieves >90% probability of target attainment (PTA). 6
- For Klebsiella pneumoniae with MIC ≤0.5 mg/L: 3 mg/kg/day achieves >90% PTA. 6
- For organisms with MIC ≥2 mg/L: Even maximum recommended doses show inadequate PTA (<60%), suggesting polymyxin B may not be optimal. 6, 7
For a 70kg patient treating typical multidrug-resistant gram-negative infections, use 1.25 mg/kg every 12 hours (87.5 mg q12h) as maintenance after the loading dose. 7
Renal Replacement Therapy Considerations
No dose adjustment is necessary for patients on continuous renal replacement therapy (CRRT)—use the standard 1.5-3 mg/kg/day maintenance dose. 1 This contrasts sharply with colistin, which requires at least 9 million IU/day during CRRT. 8
Administration Technique
Reconstitute each 500,000 unit vial in 300-500 mL of 5% dextrose and infuse over 2-3 hours. 2 Extended infusions optimize pharmacokinetic/pharmacodynamic properties. 3
Critical Safety Considerations
Polymyxin B demonstrates lower nephrotoxicity compared to colistin, making it preferable in patients with existing renal dysfunction. 1 However, monitor for:
- Nephrotoxicity (occurs in approximately 55% of patients) 7
- Neurotoxicity (occurs in approximately 55% of patients) 7
- Hyperpigmentation (less common) 7
Common Pitfall to Avoid
Do not reduce the dose based on renal function—this is the most critical error to avoid. 1, 4, 5 The older FDA labeling recommends dose reduction from 15,000 units/kg downward for renal impairment, but this recommendation contradicts all contemporary pharmacokinetic evidence showing renal function does not affect polymyxin B clearance. 2, 4, 5
When Polymyxin B Should Be Reserved
Use polymyxin B only for multidrug-resistant gram-negative infections resistant to all beta-lactams, fluoroquinolones, and carbapenems, and only in settings with local expertise in polymyxin use. 3, 9 It should never be first-line therapy. 9