Mastocytosis — SCE Dermatology MCQ

Correct answer: A — Possible systemic mastocytosis requiring further investigation

A. Possible systemic mastocytosis requiring further investigation is correct. A tryptase drawn acutely during a reaction reflects transient mast cell degranulation, but a persistently raised baseline level, sampled at least 24 hours after any acute episode when the patient is asymptomatic, points to an underlying clonal mast cell disorder rather than a one off allergic event. In UK practice, mast cell tryptase is used to confirm anaphylaxis acutely, with samples taken immediately and again within 1 to 2 hours of symptom onset, and a baseline level taken after resolution for comparison. A baseline level persistently above the reference range (classically above 20 ng/mL, or lower thresholds now proposed in some contexts) is a recognised minor criterion for systemic mastocytosis and should prompt referral for bone marrow assessment, KIT D816V mutation testing and haematology review, not simple reassurance about urticaria alone. Why the other options are wrong: E. Normal finding in all urticaria patients: baseline tryptase is normal in the great majority of patients with chronic spontaneous urticaria, so a persistently elevated level is an outlier that needs explaining, not an expected feature. B. Allergic rhinitis: this reflects IgE mediated mucosal inflammation, not a clonal or expanded mast cell burden, so baseline tryptase is not raised. D. Food allergy: food allergic reactions cause a transient rise in tryptase during the acute event, but the baseline level between reactions remains normal unless there is coexisting mastocytosis. C. Drug reaction: drug induced anaphylaxis similarly produces an acute tryptase rise that settles to a normal baseline, and does not itself cause persistent baseline elevation. Key point: a raised acute tryptase confirms mast cell degranulation, but a persistently raised baseline tryptase (taken well after any reaction) signals possible clonal mast cell disease such as systemic mastocytosis and warrants further work up.

Reference: North West London Pathology / NICE CG134 (Anaphylaxis: assessment and referral after emergency treatment): mast cell tryptase testing, acute vs baseline sampling, and tryptase >20 ug/L as a diagnostic criterion for mastocytosis. https://www.nwlpathology.nhs.uk/test/tryptase-mast-cell-tryptase-serum/