D3.266 - Component-Resolved Delayed Hypersensitivity to Piperacillin/Tazobactam and Clavulanic Acid Despite Structural Divergence

Beta-lactamase inhibitors are increasingly used in combination regimens for severe infections. Although hypersensitivity to beta-lactam combinations has traditionally been attributed to the penicillin component, inhibitors may act as independent allergens. Piperacillin, a ureidopenicillin derived from the penicillanic nucleus, and clavulanic acid, characterized by an oxazolidine ring and lacking the typical acyl side chain of penicillins, display marked structural divergence, making true immunological cross-reactivity unlikely and supporting independent sensitization in delayed reactions.

A 62-year-old woman developed a generalized pruritic maculopapular exanthem two days after discontinuation of piperacillin/tazobactam prescribed for postoperative wound infection. Laboratory tests showed eosinophilia (1200/µL). Skin biopsy findings were compatible with drug-induced toxicoderma.

Skin prick tests were negative to amoxicillin, clavulanate and piperacillin/tazobactam.

Delayed intradermal testing demonstrated positive reactions to piperacillin/tazobactam (10×10 mm induration with erythema) and clavulanate (15×15 mm), with wheal increase ≥3 mm compared to the initial bleb at 24, 48 and 96 hours, while remaining negative to amoxicillin.

Patch tests were positive to both piperacillin/tazobactam and clavulanate at 24, 48 and 96 hours, and negative to amoxicillin.

A graded oral amoxicillin challenge was tolerated.

This case demonstrates reproducible component-resolved delayed hypersensitivity to both piperacillin/tazobactam and clavulanic acid despite their structural divergence, strongly supporting independent T-cell–mediated immune mechanisms rather than cross-reactivity. All beta-lactamase inhibitors have currently been contraindicated. Further evaluation is planned to refine the sensitization profile and guide future antimicrobial strategies. Systematic testing of both antibiotic and inhibitor components is essential in delayed cutaneous reactions to avoid incomplete diagnosis and unnecessary antimicrobial restriction.