Harrison's PodClass: Internal Medicine Cases and Board Prep Ep 130: A 27-Year-Old with Penicillin Allergy
Jan 23, 2025
Dive into the complexities of penicillin allergies, focusing on desensitization therapy for cases with confirmed allergies. The podcast discusses the stigma surrounding these allergies and the importance of 'delabeling' those at low risk. It also confronts common misconceptions about allergic reactions, differentiating between immediate and delayed responses. A compelling case involving a pregnant woman with primary syphilis highlights the tough decisions healthcare providers face when treating allergic patients. Discover the diagnostic challenges and testing methods for accurately identifying penicillin allergies.
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Pregnant Patient With Penicillin Allergy
- A 27-year-old pregnant woman at eight weeks has primary syphilis and reports a childhood penicillin allergy with an ER epinephrine treatment.
- The case sets up the dilemma because penicillin is the only approved fetal-effective treatment for syphilis in pregnancy.
Allergy Terminology And Mechanisms
- Penicillin 'allergy' is often imprecise and includes immediate (within 1–6 hours) and delayed (after 6 hours) reactions.
- Immediate reactions (type I) involve mast cell/basophil activation; delayed reactions include type II–IV mechanisms and range from mild rashes to severe SJS/TEN or DRESS.
Limits Of Penicillin IgE Testing
- Serum-specific IgE testing for penicillin has low sensitivity and high specificity, so negative tests don't rule out allergy but positives are meaningful.
- Skin testing is reliable: anaphylaxis to penicillin has not been reported in skin-test-negative patients.
