Penicillin (beta-lactam) allergy
Penicillin allergy is the most commonly reported drug allergy, yet about 85 to 90 percent of labeled patients are not truly allergic and tolerate penicillin after evaluation. True IgE-mediated allergy causes urticaria, angioedema, and anaphylaxis; diagnosis uses skin testing and graded challenge to confirm or delabel, acute reactions are treated by type (epinephrine for anaphylaxis), and cephalosporin cross-reactivity is low.
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What this article covers
- Overview
Definition: penicillin allergy is a hypersensitivity reaction to penicillin or related beta-lactams, most importantly the immediate IgE-mediated form that can cause anaphylaxis.
- Epidemiology
Reported by about 10 percent of patients, but roughly 85 to 90 percent of these tolerate penicillin after evaluation.
- Risk factors
Prior immediate reaction to a beta-lactam: the key historical risk factor.
- Pathophysiology
IgE-mediated (immediate): penicillin haptenates proteins to form major and minor antigenic determinants, and specific IgE on mast cells triggers degranulation on re-exposure.
- Clinical features
Immediate reactions: urticaria, angioedema, bronchospasm, and anaphylaxis within an hour of a dose.
- Differential diagnosis
Non-allergic drug intolerance: nausea or diarrhea reported as "allergy," without urticaria, angioedema, or anaphylaxis.
- Diagnosis
Establish or exclude with penicillin skin testing followed by a graded oral challenge: the diagnostic basis is validated penicillin skin testing to the major and minor determinants, and, when…
- Management
First, treat an active reaction by type: for anaphylaxis give epinephrine 0.3 to 0.5 mg IM into the anterolateral thigh, repeated every 5 to 15 minutes, with airway support and fluids; for urticaria…
- Complications
Anaphylaxis: from true immediate allergy or during challenge without proper precautions.
- Prognosis
Excellent after delabeling: most patients safely resume beta-lactams.
- Prevention
Take an accurate allergy history and document the reaction type at the time it occurs.
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Related articles
- Anaphylaxis
Anaphylaxis is an acute, potentially fatal systemic hypersensitivity reaction from massive mast cell and basophil mediator release, presenting within minutes as skin, respiratory, cardiovascular, and gastrointestinal involvement after allergen exposure. It is a clinical, treat-first diagnosis, and intramuscular epinephrine is the immediate life-saving therapy.
- Urticaria
Urticaria is transient, pruritic, edematous wheals from mast cell histamine release, classified by duration into acute (under 6 weeks) and chronic (6 weeks or longer, usually spontaneous/idiopathic). Diagnosis is clinical with minimal testing, and treatment is stepwise second-generation H1 antihistamines up-titrated as needed, adding omalizumab for refractory chronic disease.