A 28-year-old woman presents to the emergency department with a 3-day history of fever, productive cough with purulent sputum, and dyspnea. She has a history of cystic fibrosis. On examination, she is febrile (38.5°C), tachypneic (RR 28/min), and has crackles in the right lower lobe. Sputum culture grows Pseudomonas aeruginosa. She has normal renal function (creatinine 0.8 mg/dL). Which of the following beta-lactam antibiotics is most appropriate for this patient?

Explanation

## Clinical Context This patient has cystic fibrosis with Pseudomonas aeruginosa respiratory infection—a common and serious complication. The choice of beta-lactam must cover Gram-negative organisms, particularly Pseudomonas, which is intrinsically resistant to aminopenicillins and first-generation cephalosporins. ## Why Piperacillin-Tazobactam? **Key Point:** Piperacillin is an extended-spectrum (antipseudomonal) penicillin that has excellent activity against Pseudomonas aeruginosa. The addition of tazobactam (a beta-lactamase inhibitor) extends coverage to beta-lactamase-producing organisms. **High-Yield:** Antipseudomonal beta-lactams include: - Piperacillin (± tazobactam) - Ceftazidime - Cefepime - Carbapenems (meropenem, imipenem) Piperacillin-tazobactam is a first-line choice for Pseudomonas infections in cystic fibrosis patients and is preferred over carbapenems when resistance patterns permit, to preserve carbapenem stewardship. ## Spectrum Comparison Table | Beta-lactam | Gram+ | Gram− (non-Ps) | Pseudomonas | Anaerobes | Beta-lactamase stable | | --- | --- | --- | --- | --- | --- | | Amoxicillin-clavulanate | +++ | + | − | ++ | Yes (clavulanate) | | Ampicillin | +++ | + | − | − | No | | Cephalexin (1st gen) | +++ | + | − | − | No | | Piperacillin-tazobactam | ++ | +++ | +++ | +++ | Yes (tazobactam) | | Ceftazidime | + | +++ | +++ | − | No | **Clinical Pearl:** In cystic fibrosis, chronic Pseudomonas colonization/infection is a major driver of lung decline. Early aggressive treatment with antipseudomonal agents (IV piperacillin-tazobactam or ceftazidime) is standard of care. ## Dosing Note Piperacillin-tazobactam is typically dosed at 4.5 g IV every 6–8 hours (or 3.375 g every 4–6 hours for less severe infections). In cystic fibrosis, higher and more frequent dosing may be needed due to altered pharmacokinetics.