Optimizing the Diagnostic Approach to Cephalosporin Allergy Testing

Inclusion Criteria: 1. Age 18-70 years old. 2. Reaction history consistent with a potential immediate hypersensitivity reaction (pruritus, urticaria, erythema, angioedema, bronchospasm, wheezing, shortness of breath, anaphylaxis, or hypotension) to cefazolin, ceftazidime, ceftriaxone, cefepime, cephalexin, cefaclor, cefadroxil, cefuroxime, cefpodoxime, cefdinir, or cefixime. 3. English speaking or non-English speaking with translation services available. Exclusion Criteria: 1. Severe concomitant medical condition (e.g., unstable coronary artery disease, congestive heart failure, severe chronic obstructive pulmonary disease, poorly controlled asthma, chronic renal failure, cirrhosis, or end-stage liver disease.) 2. History of Clostridioides difficile infection 3. Chronic spontaneous urticaria or systemic mastocytosis 4. Incident reaction required cardiopulmonary resuscitation 5. Reaction to 2 or more cephalosporin antibiotics 6. Active infection or antibiotic treatment within 7 days 7. Treatment with systemic antihistamines or corticosteroids within 7 days 8. Treatment with omalizumab or dupilumab within 60 days 9. Significant immunosuppression 10. Treatment with a beta-blocker or ACE inhibitor within 7 days 11. Use of investigational drugs within 60 days of participation 12. Anaphylaxis in the last 30 days 13. Penicillin anaphylaxis within the past year confirmed with positive penicillin skin tests 14. Prison or jail inmates, pregnant women, severe cognitive impairment 15. Current, diagnosed, mental illness or current, diagnosed, or self-reported drug or alcohol abuse that, in the opinion of the investigator, would interfere with the participant's ability to comply with study requirements 16. Past or current medical problems or findings from physical examination or laboratory testing that are not listed above, which, in the opinion of the investigator, may pose additional risks from participation in the study, may interfere with the participant's ability to comply with study requirements or that may impact the quality or interpretation of the data obtained from the study. 17. Inability or unwillingness of a participant to give written informed consent or comply with study protocol