In an 89-year-old patient with a urinary tract infection caused by KPC-producing Klebsiella pneumoniae who remains febrile after five days of ceftazidime‑avibactam 2.5 g IV every 8 hours and after a three‑day trial of imipenem‑cilastatin added to ceftazidime‑avibactam (now discontinued), what management suggestions do you recommend?

Immediate Recommendation for Persistent Fever Despite Ceftazidime-Avibactam

Add intravenous fosfomycin to the ceftazidime-avibactam regimen immediately, and urgently verify susceptibility testing including ceftazidime-avibactam MIC and consider therapeutic drug monitoring (TDM) of ceftazidime-avibactam levels.

Critical Assessment Steps

Before changing therapy, you must:

1. Verify true treatment failure vs. alternative fever source

   • Rule out non-infectious causes (drug fever, thrombophlebitis, C. difficile)
   • Assess for undrained abscess or complicated UTI (perinephric abscess, emphysematous pyelonephritis)
   • Obtain repeat blood and urine cultures immediately
   • Check imaging if not already done (renal ultrasound or CT)

2. Review microbiological data urgently

   • Confirm KPC-type (not metallo-beta-lactamase which would be ceftazidime-avibactam resistant)
   • Request ceftazidime-avibactam MIC if available
   • Verify no aztreonam susceptibility testing was performed (you stated unavailable, but confirm)
   • Check fosfomycin susceptibility

3. Assess dosing adequacy

   • At 89 years old, verify renal function and confirm she's receiving appropriate dosing
   • Critical concern: Renal dose adjustment of ceftazidime-avibactam is independently associated with 4.5-fold increased mortality in KPC-Kp bacteremia 1
   • If creatinine clearance is reduced and she's receiving less than 7.5g daily, this may explain treatment failure

Primary Treatment Recommendation

Add IV fosfomycin to ceftazidime-avibactam based on the following rationale:

• Fosfomycin demonstrates synergistic in vitro activity against carbapenem-resistant K. pneumoniae 2
• Fosfomycin-containing combinations showed reduced mortality (RR 0.55) compared to other regimens for CRKP infections 2
• One Italian study specifically showed that ceftazidime-avibactam plus fosfomycin reduced secondary KPC-Kp infections compared to ceftazidime-avibactam with other agents 2
• Fosfomycin susceptibility rates in CRKP range 39-99%, making it a reasonable empiric addition 2
• IV fosfomycin is generally safe with mild adverse reactions; main concern is reversible hypokalemia (monitor potassium closely) 2

Dosing: High-dose IV fosfomycin (typically 6-8g every 8 hours for serious infections, adjusted for renal function)

Why Imipenem-Cilastatin Failed and Should Not Be Restarted

The imipenem-cilastatin combination was appropriately discontinued because:

• In an 89-year-old, carbapenems carry significant seizure risk, especially with renal impairment 3
• KPC-producing organisms are carbapenem-resistant by definition
• The "double carbapenem" strategy (imipenem + ceftazidime-avibactam) lacks strong evidence in this setting
• Continued carbapenem exposure without benefit increases toxicity risk without improving outcomes

Alternative Considerations if Fosfomycin Unavailable or Resistant

If fosfomycin cannot be obtained or organism is resistant:

1. Aminoglycoside addition (gentamicin or amikacin)

   • Check susceptibility first
   • Use with extreme caution at age 89 due to nephrotoxicity
   • Mandatory TDM to optimize dosing and reduce toxicity 2
   • Consider single daily dosing with peak/trough monitoring

2. Tigecycline (if susceptible)

   • Achieves good urinary concentrations
   • Often retained activity against KPC-producers
   • Lower mortality risk than polymyxins in elderly

3. Colistin (last resort only)

   • High nephrotoxicity risk in 89-year-old
   • Requires TDM for optimal dosing 2
   • Reserve only if all other options exhausted

Critical Monitoring

• Therapeutic drug monitoring: Request ceftazidime-avibactam levels if available; subtherapeutic levels may explain failure 2
• Daily renal function: Especially if adding aminoglycoside or colistin
• Repeat cultures: At 48-72 hours after regimen change
• Potassium levels: If using fosfomycin (risk of hypokalemia) 2

Special Geriatric Considerations

In this 89-year-old patient 4:

• Ensure adequate hydration status
• Monitor for delirium (may be infection-related, not just fever)
• Assess functional status and goals of care
• Consider whether asymptomatic bacteriuria was mistaken for UTI initially (though persistent fever suggests true infection)
• Drug interactions with polypharmacy are likely—review all medications

What NOT to Do

• Do not use fluoroquinolones: Generally inappropriate in frail elderly with multiple comorbidities and high resistance rates 4
• Do not discontinue ceftazidime-avibactam: It remains the backbone therapy for KPC-producers 5, 6
• Do not restart imipenem: Failed trial, seizure risk in elderly 3
• Do not delay source control: If imaging shows drainable collection, antibiotics alone will fail

Expected Timeline

• Clinical improvement (defervescence) should occur within 48-72 hours of appropriate combination therapy
• If still febrile at 72 hours post-fosfomycin addition, strongly consider:
  • Resistance emergence (repeat susceptibilities) 7
  • Alternative diagnosis
  • Inadequate source control

The combination of ceftazidime-avibactam plus fosfomycin represents the most evidence-based escalation strategy for this clinical scenario, with acceptable safety profile in an elderly patient and demonstrated efficacy in CRKP infections 2.