A 32-year-old woman develops sudden-onset anaphylaxis 15 minutes after receiving intravenous penicillin for pneumonia. She has no prior history of penicillin allergy. Which investigation is most appropriate to confirm the immunological basis of this adverse drug reaction?

Explanation

## Diagnosis of Immediate Hypersensitivity Reactions (Type I ADR) ### Clinical Context The patient presents with anaphylaxis (immediate-onset, within 15 minutes of drug exposure). This is a **Type I hypersensitivity** adverse drug reaction mediated by IgE-mast cell degranulation. ### Investigation of Choice: Serum Tryptase **Key Point:** Serum tryptase is the gold standard acute-phase marker for mast cell degranulation and anaphylaxis diagnosis. - **Timing:** Must be drawn within **15–30 minutes** of symptom onset (peak levels) - **Mechanism:** Tryptase is a preformed enzyme released from mast cell granules during degranulation - **Sensitivity & Specificity:** ~90% sensitive for anaphylaxis when drawn in the acute window - **Advantage:** Objective, quantitative, and independent of clinical assessment variability **High-Yield:** Serum tryptase ≥11.4 ng/mL (or >20% above baseline) within 15–30 min strongly supports anaphylaxis diagnosis. ### Why This Timing Matters | Investigation | Optimal Timing | Sensitivity in Anaphylaxis | Clinical Use | |---|---|---|---| | **Serum tryptase** | 15–30 min post-onset | ~90% | Acute confirmation | | **Skin prick test** | ≥2–4 weeks later | ~70–80% | Allergen identification (delayed) | | **Intradermal test** | ≥2–4 weeks later | Variable, risk of re-reaction | Confirmatory (delayed) | | **RAST/Specific IgE** | Any time | ~60–70% | Supportive (not acute) | **Clinical Pearl:** Skin testing and RAST are performed **after acute phase resolution** (2–4 weeks) to avoid re-triggering anaphylaxis and to allow IgE levels to stabilize. ### Why Serum Tryptase is Superior in Acute Setting 1. **Immediate availability:** Can be sent stat from bedside 2. **No risk of re-reaction:** Non-provocative, in vitro test 3. **Objective marker:** Not dependent on observer bias 4. **Diagnostic window:** Acute elevation is pathognomonic for mast cell activation **Mnemonic:** **TRYPTASE = T (Timing critical), R (Rapid mast cell marker), Y (Yes to anaphylaxis), P (Peak 15–30 min), T (Type I hypersensitivity), A (Acute phase only), S (Specific for degranulation), E (Elevated ≥11.4 ng/mL)** ### Post-Acute Confirmation (After 2–4 Weeks) Once the acute phase resolves, skin prick testing or RAST can identify the specific penicillin determinant responsible for sensitization.