A 28-year-old woman presents with acute onset dyspnea, urticaria, and angioedema 15 minutes after receiving intravenous penicillin for a urinary tract infection. She has a history of penicillin allergy. Which investigation is most appropriate to confirm the diagnosis of IgE-mediated hypersensitivity reaction?

Explanation

## Diagnosis of IgE-Mediated Hypersensitivity (Type I) ### Clinical Context The patient presents with classic features of anaphylaxis: rapid onset (15 minutes), urticaria, angioedema, and dyspnea following drug exposure in a sensitized individual. ### Why Serum Tryptase is the Investigation of Choice **Key Point:** Serum tryptase is the most specific and practical marker of mast cell degranulation in acute IgE-mediated hypersensitivity reactions. **High-Yield:** Tryptase levels: - Peak at 15 minutes to 3 hours after symptom onset - Remain elevated for up to 5–6 hours - Baseline (pre-event) levels are typically < 11.4 ng/mL - A rise of ≥ 1.2-fold above baseline is diagnostic of mast cell activation - Most sensitive and specific during the acute phase ### Timing and Interpretation | Investigation | Timing | Sensitivity | Specificity | | --- | --- | --- | --- | | **Serum tryptase** | 15 min–3 hrs | High (acute phase) | High | | Skin prick test | After recovery (2–4 weeks) | Moderate | Moderate | | Histamine | Rapid clearance (minutes) | Low (missed window) | Moderate | | Complement levels | Not specific | N/A | Low | **Clinical Pearl:** In anaphylaxis, tryptase should be drawn immediately upon arrival and again at 24 hours (baseline). The acute sample is diagnostic; the baseline helps confirm the rise. ### Mechanism Mast cell degranulation releases preformed mediators including tryptase, histamine, and leukotrienes. Tryptase is a neutral protease stored in mast cell granules and is released proportionally to the degree of degranulation. **Mnemonic: MAST (Mast cell markers)** — **M**ediators (histamine, tryptase), **A**cute phase (tryptase peaks 15 min–3 hrs), **S**pecific (tryptase > histamine), **T**iming (draw early). ### Why Other Tests Are Not First-Line Skin prick testing, while useful for confirmation, must be deferred until 2–4 weeks after the acute event (to avoid re-triggering anaphylaxis and because skin reactivity may be temporarily suppressed). Complement levels are relevant in Type II and Type III hypersensitivity, not Type I. Histamine is rapidly cleared and has a narrow window of detection (minutes). [cite:Robbins 10e Ch 6]