Take the antibiotic allergy history.
About 1 in 10 patients carry a penicillin-allergy label; more than 90% are not allergic on formal evaluation. The history is what separates them.
Reviewed August 2026 · verify against current guidelines
Take the antibiotic allergy history.
About 1 in 10 patients carry a penicillin-allergy label. More than 90% are not allergic on formal evaluation. The history is what separates them.
The first question — how soon after the dose?
| Timing | Mechanism | Pattern |
|---|---|---|
| Minutes to 1 hour | IgE | Urticaria, angioedema, wheeze, hypotension, vomiting. Re-exposure can kill — this is the history that needs testing. |
| 6 hours to 6 weeks | T cell | Maculopapular rash, SJS/TEN, DRESS, AGEP. Skin testing does not apply. A blistering rash? Never again. |
Then score it — PEN-FAST
| Points | Criterion |
|---|---|
| 2 | Five years or less since the reaction |
| 2 | Anaphylaxis or angioedema, or a severe cutaneous reaction |
| 1 | Treatment was required for the reaction |
PEN-FAST score → risk of a positive test
| 0 | 1–2 | 3 | 4–5 |
|---|---|---|---|
| 0.6% | 5% | 20% | 50% |
- PEN-FAST <3: consider supervised direct oral challenge when appropriate, usually without prior skin testing, per local protocol.
- PEN-FAST ≥3: consider allergy specialist assessment.
What actually cross-reacts
| Agent | Cross-reactivity |
|---|---|
| Aminocephalosporins with identical side chains (eg, after amoxicillin or ampicillin) | 16.45% |
| Cephalosporins with low side-chain similarity | 2.11% |
| Carbapenems (meropenem, ertapenem, imipenem) | 0.87% |
Record it, do not just repeat it
- Drug, dose and date.
- What happened, in the patient's words.
- How long after the dose, in hours.
- Which beta-lactams they have tolerated since.
Khan et al, J Allergy Clin Immunol 2022; Trubiano et al, JAMA Intern Med 2020; Copaescu et al, JAMA Intern Med 2023; Picard et al, J Allergy Clin Immunol Pract 2019. Evidence is derived predominantly from North American, Australian, and European populations. Educational only — verify against local protocol.
Sources
Verify against current guidelines and local protocol before acting.
- Khan, J Allergy Clin Immunol 2022 · Trubiano, JAMA Intern Med 2020 (PEN-FAST) · Copaescu, JAMA Intern Med 2023 · Picard, J Allergy Clin Immunol Pract 2019
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