# Mastocytosis

> Mastocytosis for NEET-PG Dermatology covering urticaria pigmentosa, the Darier sign, KIT D816V, serum tryptase and advanced disease treatment.

- Canonical URL: https://prepelephant.com/topics/neet-pg/dermatology/mastocytosis
- Exam / course: NEET-PG · Subject: Dermatology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Rook's Textbook of Dermatology; Fitzpatrick's Dermatology; Harrison's Principles of Internal Medicine
- How to cite: "Mastocytosis", PrepElephant, https://prepelephant.com/topics/neet-pg/dermatology/mastocytosis

## Direct answer

Mastocytosis is a clonal proliferation of mast cells, most often driven by an activating KIT D816V mutation in adults, and it presents as cutaneous lesions such as urticaria pigmentosa or as systemic disease. Urticaria pigmentosa shows brown macules or papules that urticate when stroked, a finding called the Darier sign. Antihistamines and mediator avoidance control most symptoms, while midostaurin or avapritinib are reserved for advanced systemic disease.

## What you must remember

- **Cutaneous forms:** urticaria pigmentosa is the commonest form in children; diffuse cutaneous mastocytosis presents in infants with thickened skin and bullae; telangiectasia macularis eruptiva perstans is seen in adults.
- **Darier sign:** an urticarial wheal and flare when a lesion is stroked or rubbed, reflecting mast cell degranulation.
- **Mediator symptoms:** flushing, pruritus, abdominal cramps, diarrhoea, syncope and anaphylaxis, often triggered by heat, friction, alcohol, opioids or NSAIDs.
- **Genetics:** KIT encodes the stem cell factor receptor, and the activating KIT D816V mutation is found in most adults with systemic mastocytosis.
- **Biomarker:** a baseline serum tryptase above 20 ng/mL is a WHO minor criterion for systemic disease.
- **Systemic organs:** bone marrow, liver, spleen, lymph nodes and gastrointestinal tract are the usual sites of systemic involvement.
- **Management:** H1 antihistamines first, with H2 antagonists and sodium cromoglycate added as needed, an adrenaline autoinjector for anaphylaxis, and midostaurin or avapritinib for advanced disease.

## A pathway from skin lesion to systemic work-up

Mastocytosis is missed when skin lesions are dismissed as pigmentation or eczema. The pathway below follows the usual route from bedside signs to systemic screening.

1. **Look for the lesions:** brown macules or papules that urticate on stroking, most often on the trunk and limbs of children.
2. **Confirm the Darier sign:** rubbing or stroking a lesion produces a wheal and flare, reflecting mast cell degranulation.
3. **Take a symptom history:** ask about flushing, itch, abdominal cramps, syncope and previous anaphylaxis, which reflect mediator release.
4. **Biopsy a lesion:** a skin biopsy stained for tryptase or CD117 shows dense mast cell infiltration.
5. **Measure serum tryptase:** a baseline level above 20 ng/mL supports systemic disease and is a WHO minor criterion.
6. **Test for KIT D816V:** a sensitive assay in blood or marrow confirms clonal disease, and bone marrow biopsy assesses systemic involvement.
7. **Grade the disease:** classify as indolent, aggressive or associated with another haematological neoplasm, which determines whether advanced therapy is needed.

## Where the exam tries to catch you

Mastocytosis is easy to miss because its skin signs look like pigmentation, and its systemic form can be serious.

- **Trap:** treating urticaria pigmentosa as simple post-inflammatory pigmentation. **Correct:** brown lesions that urticate on stroking are mastocytosis, confirmed by the Darier sign.
- **Trap:** assuming all mastocytosis is cutaneous and benign. **Correct:** systemic mastocytosis needs tryptase, KIT D816V testing and marrow assessment, because bone and gastrointestinal involvement can be serious.
- **Trap:** using imatinib for KIT D816V-positive disease. **Correct:** D816V confers resistance to imatinib, so midostaurin or avapritinib are the targeted options in advanced disease.
- **Trap:** giving NSAIDs or opioids freely for pain. **Correct:** these are common triggers of mediator release, so they are avoided or used with caution.

## Frequently asked questions

### What is the Darier sign in mastocytosis?

The Darier sign is the development of an urticarial wheal and flare when a mastocytosis lesion is stroked or rubbed. It reflects mast cell degranulation within the lesion and is a useful bedside sign in urticaria pigmentosa.

### Which mutation is seen in most adults with systemic mastocytosis?

A KIT D816V activating mutation is found in most adults with systemic mastocytosis. The mutation makes the KIT receptor constitutively active and resists imatinib, which matters for treatment choice.

### What serum tryptase level supports systemic mastocytosis?

A baseline serum tryptase above 20 ng/mL is a minor diagnostic criterion for systemic mastocytosis in the WHO classification. A raised level alongside skin lesions and KIT D816V positivity supports systemic disease and calls for bone marrow review.

### Which drugs are used for advanced systemic mastocytosis?

Midostaurin and avapritinib are used for advanced systemic mastocytosis, with avapritinib also used in some indolent forms. Antihistamines and sodium cromoglycate control symptoms in most patients, and an adrenaline autoinjector is prescribed for those at risk of anaphylaxis.

### What triggers mast cell mediator release in mastocytosis?

Heat, friction, alcohol, opioids such as codeine, and NSAIDs such as aspirin are common triggers of mediator release in mastocytosis. Patients are advised to avoid these triggers where possible and to carry emergency medication when it is prescribed.