A 38-year-old woman with a history of severe penicillin anaphylaxis presents with septic shock secondary to a urinary tract infection. Blood cultures are pending. Which is the preferred empirical antibiotic regimen in this patient?

Explanation

## Empirical Sepsis Treatment in Beta-Lactam-Allergic Patients **Key Point:** In patients with true IgE-mediated (anaphylaxis) penicillin allergy, beta-lactams (including cephalosporins and carbapenems) must be avoided. Aztreonam is a **monobactam** with a unique beta-lactam ring structure that has minimal cross-reactivity with penicillins. ### Why Aztreonam + Vancomycin? | Feature | Aztreonam | Cephalosporins | Carbapenems | |---------|-----------|----------------|-------------| | **Cross-reactivity with penicillin** | <1% (safe in anaphylaxis) | 1–3% (relative contraindication) | ~1% (relative contraindication) | | **Gram-negative coverage** | Excellent (E. coli, Klebsiella, Proteus) | Excellent | Excellent | | **MRSA coverage** | No | No | No | | **Use in penicillin anaphylaxis** | **Safe** | Avoid | Avoid | **Clinical Pearl:** Aztreonam's monobactam structure is structurally distinct from the penicillin beta-lactam ring, resulting in <1% cross-reactivity even in severe penicillin allergy. It is the preferred beta-lactam alternative. ### Combination Rationale - **Aztreonam**: Covers gram-negative organisms (UTI pathogens: E. coli, Klebsiella) - **Vancomycin**: Covers gram-positive organisms and MRSA **High-Yield:** Aztreonam has **no activity against gram-positive organisms or anaerobes**; vancomycin is mandatory as a partner agent. **Mnemonic:** **AZTEC** = **Aztreonam** is **The** **Excellent** **Choice** in beta-lactam anaphylaxis (monobactam = minimal cross-reactivity).