Empiric Treatment for CRAB Pneumonia in Australia
For empiric treatment of carbapenem-resistant Acinetobacter baumannii (CRAB) pneumonia in Australia, use intravenous polymyxin B or colistin combined with at least one additional agent (high-dose extended-infusion meropenem if MIC ≤32 mg/L, or sulbactam/ampicillin-sulbactam), plus adjunctive inhaled colistin. 1, 2
Core Empiric Regimen Structure
First-Line Combination Therapy
The foundation of empiric CRAB treatment requires intravenous polymyxin-based combination therapy rather than monotherapy 1, 2. The evidence strongly supports combinations over monotherapy, with moderate-quality evidence showing reduced mortality, treatment failure, and eradication failure 2.
Recommended empiric combinations:
IV polymyxin B or colistin (strong recommendation) 1
- Colistin methanesulfonate (CMS): Loading dose followed by maintenance dosing based on renal function
- Polymyxin B: 15,000-25,000 units/kg/day divided every 12 hours
PLUS one of the following:
- High-dose extended-infusion meropenem (2g over 3 hours every 8 hours) if suspected CRAB MIC ≤32 mg/L 2
- Ampicillin-sulbactam (9g sulbactam component daily via extended infusion) 1
- High-dose tigecycline (loading 200mg, then 100mg every 12 hours) - though guidelines recommend against tigecycline for HAP/VAP 1, newer evidence suggests high-dose regimens may have a role 3
PLUS adjunctive inhaled colistin (weak recommendation, but supported) 1, 2
- 1-2 million units every 8-12 hours via nebulizer
- Reduces mortality and clinical failure rates by approximately 14-18% 2
Critical Implementation Points
Dosing Conversions for Polymyxins
Pay careful attention to dose units 2:
- Colistin methanesulfonate: 1 million units = 80mg CMS mass = 33mg colistin base activity (CBA)
- Polymyxin B sulfate: 1mg = 10,000 units
- Use CBA for dosing calculations to avoid errors
When to Use Carbapenem in the Combination
Despite carbapenem resistance, include high-dose extended-infusion meropenem when 2:
- Meropenem MIC ≤32 mg/L for CRAB
- Administered as 2g over 3 hours every 8 hours
- The polymyxin-carbapenem combination shows synergistic activity even against "resistant" isolates at these MIC thresholds
Monitoring Requirements
- Renal function: Monitor creatinine daily during polymyxin therapy 2
- Therapeutic drug monitoring (TDM): Strongly encouraged for polymyxins where available 2
- Avoid concurrent nephrotoxic/ototoxic agents with polymyxins 2
- Clinical response assessment: If CPIS >6 on day 3 of treatment, mortality risk is significantly elevated 4
Alternative Agents (If Available in Australia)
Newer Options
While not specifically mentioned for empiric use in Australian guidelines, if available and susceptibility known:
Sulbactam-durlobactam: Emerging as highly effective, prioritized in recent IDSA guidance 5, 6, 7, 8
- 1g/1g infused over 3 hours every 6 hours
- Can be used with or without carbapenem
Cefiderocol: Consider for refractory cases 5, 6, 3
- Continuous infusion preferred for pneumonia
- Mortality data inconsistent in trials
Common Pitfalls to Avoid
Do NOT use polymyxin monotherapy - significantly higher mortality rates (66.7% vs 52.8% at 30 days with combination) 4
Do NOT use tigecycline as primary agent for HAP/VAP - strong recommendation against 1, though may have role in combination at high doses
Do NOT add rifampicin routinely - guidelines suggest against adjunctive rifampicin with colistin 1
Do NOT underdose polymyxins - use appropriate loading doses and adjust for renal function
Do NOT delay inhaled colistin - add from the start, not after IV therapy fails 2
Risk Stratification
High-risk patients requiring aggressive combination therapy:
- Septic shock at presentation
- APACHE II score ≥20
- Severe sepsis
- Ventilator-associated pneumonia
These patients have 30-day mortality rates of 53-67% and require full triple therapy (IV polymyxin + second agent + inhaled colistin) 4.
Empiric monotherapy is associated with significantly worse outcomes (OR 1.63 for 30-day mortality) and should be avoided 4.