A 32-year-old woman with a history of penicillin allergy presents with a urinary tract infection caused by E. coli. She reports a rash after taking amoxicillin 5 years ago. Before prescribing a beta-lactam antibiotic, which investigation is most appropriate to determine if she can safely receive a cephalosporin?

## Investigation of Choice: Allergy Testing Before Beta-Lactam Use ### Clinical Context The patient reports a prior rash to amoxicillin (a penicillin). Before prescribing a cephalosporin—which shares a beta-lactam ring with penicillins—allergy testing is essential to assess cross-reactivity risk and guide safe antibiotic selection. ### Why Skin Prick Test is Correct **Key Point:** Skin prick testing (SPT) with penicilloyl-polylysine (major determinant) and minor determinants (ampicillin, amoxicillin, or benzylpenicillin) is the gold standard for diagnosing IgE-mediated (Type I) penicillin hypersensitivity. **High-Yield:** - A negative SPT has a **negative predictive value >99%** for immediate hypersensitivity reactions. - If SPT is negative, the patient can safely receive cephalosporins (cross-reactivity risk is <2% with 3rd/4th generation cephalosporins). - If SPT is positive, cephalosporins should be avoided; alternatives (fluoroquinolones, macrolides) are preferred. **Clinical Pearl:** The rash reported 5 years ago is likely a delayed (non-IgE-mediated) reaction if it occurred >1 hour after ingestion. SPT will be negative in such cases, and cephalosporins are safe. However, SPT is still the investigation of choice to rule out concurrent IgE-mediated sensitization. ### Mechanism SPT detects mast cell degranulation triggered by IgE-allergen cross-linking, providing rapid (15–20 min) results with high sensitivity and specificity for immediate hypersensitivity. [cite:KD Tripathi 8e Ch 52]