Allergy: Diagnosis and Management
Core concepts
Allergic disease is best understood as an IgE-mediated (or, in some conditions, non-IgE or mixed) hypersensitivity response to otherwise harmless antigens. Candidates must distinguish sensitization (a positive test showing specific IgE, with or without symptoms) from clinical allergy (symptoms reproducibly tied to exposure). Know the mechanisms and limits of each test: skin prick testing detects specific IgE on mast cells and has high sensitivity but variable specificity; serum specific IgE has similar operating characteristics; intradermal testing is reserved for select drug and venom evaluations because of false positives and reaction risk. Total IgE is rarely diagnostic.
Management framework
Treatment follows a stepwise logic: avoidance where feasible, pharmacologic control (intranasal corticosteroids are first-line for allergic rhinitis; antihistamines are adjuncts), and immunotherapy for appropriately selected patients. Epinephrine is first-line for anaphylaxis, given intramuscularly into the lateral thigh, with patients observed for biphasic reactions. Asthma must be controlled before immunotherapy is initiated.
Traps
Questions love to test the sensitization-versus-allergy distinction, the fact that a positive test never establishes diagnosis alone, and the rule that skin testing must be interpreted with history. Another classic trap: confusing first-line with adjunctive therapy (e.g., antihistamines for rhinitis). Watch for timing questions (epinephrine dosing interval, observation periods) and for distractors that recommend oral provocation without appropriate safety conditions.
Worked rule of thumb
A patient with seasonal sneezing and a positive grass pollen skin test has allergic rhinitis if symptoms track the season. If the same patient has a positive test but no symptoms with exposure, that is sensitization, not disease, and treatment is not indicated. Test only to confirm a history, never to screen.