IV Fosfomycin Dosing
For adults with normal renal function, administer IV fosfomycin 15-24 g/day divided every 6-8 hours, with the specific dose and interval determined by the pathogen's MIC and infection severity.
Adult Dosing by Pathogen and Clinical Scenario
Standard Dosing for Multidrug-Resistant Gram-Negative Infections
For Klebsiella pneumoniae (MIC ≤64 mg/L):
- 6 g every 8 hours (18 g/day) for critically ill patients 1
- This achieves adequate pharmacodynamic targets (AUC/MIC >28.2) for most KP infections
For Pseudomonas aeruginosa:
- 6 g every 6 hours (24 g/day) for critically ill patients 1
- Requires more frequent dosing due to higher PD target (AUC/MIC >40.8) 2
For Escherichia coli:
- 15 g/day divided every 8 hours for patients with normal kidney function 2
- This regimen achieves 97.1% probability of target attainment against E. coli
- Note: Higher doses (18-24 g/day) may be needed for MIC >32 mg/L 1
High-Dose Regimens for Severe Infections
For carbapenem-resistant organisms with high MICs (64 mg/L):
- 18-24 g/day divided every 6-8 hours 1
- Clinical data supports high-dose IV fosfomycin for KPC-producing Klebsiella pneumoniae UTIs 3
Renal Impairment Adjustments
Critical consideration: Fosfomycin is renally eliminated, and dosing must be adjusted based on creatinine clearance 4, 2.
Dosing by Creatinine Clearance:
- CrCl 91-120 mL/min: 15 g/day (standard dosing) 2
- CrCl 54-90 mL/min: Reduce dose proportionally based on kidney function
- CrCl <54 mL/min: Significant dose reduction required; half-life increases from 11 to 50 hours 4
Continuous Renal Replacement Therapy (CRRT):
For patients on CVVH:
- 8 g every 12 hours 5
- CVVH removes approximately 76.7% of the drug 5
- This maintains therapeutic levels equivalent to standard dosing in patients with intact renal function
For patients on PIRRT:
- 8 g every 8 hours (24 g/day) for KP infections (MIC 64 mg/L) 1
- 7 g every 8 hours for PA infections 1
- 5 g every 8 hours may be adequate depending on dialysis intensity 6
- Important caveat: Drug accumulation can occur depending on dialysis frequency and intensity; monitor closely 6
Pediatric Dosing
Dosing is primarily weight-based and age-dependent 7:
Neonates and Infants (0-12 months):
- Pre-term neonates (<40 weeks postmenstrual age): 100 mg/kg/day divided every 12 hours
- Term neonates and infants: 100-200 mg/kg/day divided every 8 hours
- Immature neonates may use 12-hour intervals, but 6-8 hour intervals are preferred for older infants 7
Children (1-12 years):
- 200-400 mg/kg/day divided every 6-8 hours 7
- Current recommendations may be insufficient for pathogens with MIC ≥32 mg/L in children with normal renal function 7
- Prefer 6-8 hour dosing intervals over 12 hours for optimal time above MIC 7
Key Clinical Considerations
Dosing interval matters: Fosfomycin is a time-dependent antibiotic; maintaining time above MIC (T>MIC) of 40-70% is critical for efficacy 7. Shorter intervals (every 6-8 hours) are superior to every 12 hours except in neonates.
MIC-driven dosing: For organisms with MIC >32 mg/L, standard doses are often inadequate. Consider 18-24 g/day in adults 1.
Renal function monitoring: The half-life dramatically increases with renal impairment (from 5.7 hours to 50 hours in severe impairment) 4. Failure to adjust can lead to toxicity.
CRRT variability: Dialysis intensity (L/h) significantly affects drug removal. A 1.5 L/h intensity removes substantial fosfomycin, requiring aggressive dosing 6.
Susceptibility testing limitations: For KPC-producing organisms, in vitro susceptibility may not correlate with clinical outcomes when using high-dose regimens 3. Clinical cure rates of 70% were achieved regardless of susceptibility results in UTI patients.