WHO diagnostic criteria for cutaneous and systemic mastocytosis

Cutaneous mastocytosis (CM)

Skin lesions demonstrating the typical findings of urticaria pigmentosa/maculopapular cutaneous mastocytosis, diffuse cutaneous mastocytosis or solitary mastocytoma, and typical histologic infiltrates of mast cells in a multifocal or diffuse pattern in an adequate skin biopsy.* In addition, features/criteria sufficient to establish the diagnosis of systemic mastocytosis must be absent.^[1-3] There are 3 variants of cutaneous mastocytosis (refer to UpToDate table on the classification of mastocytosis variants).

Systemic mastocytosis (SM)

The diagnosis of systemic mastocytosis can be made when the major criterion and at least 1 minor criterion are present, or when ≥3 minor criteria are present.

Criterion

Major criterion:

Multifocal dense infiltrates of mast cells (≥15 mast cells in aggregates) detected in sections of bone marrow and/or other extracutaneous organ(s).

Minor criteria:

In biopsy sections of bone marrow or other extracutaneous organs, >25% of the mast cells in the infiltrate are spindle-shaped or have atypical morphology or >25% of all mast cells in bone marrow aspirate smears are immature or atypical.
Detection of an activating point mutation at codon 816 of KIT in the bone marrow, blood, or another extracutaneous organ.
Mast cells in bone marrow, blood, or another extracutaneous organ express CD25, with or without CD2, in addition to normal mast cell markers.^¶
Serum total tryptase is persistently >20 ng/mL, unless there is an associated myeloid neoplasm, in which case this parameter is not valid.

WHO: World Health Organization.

* This criterion applies to both the dense focal area and the diffuse mast cell infiltrates in the biopsy.
¶ CD25 is the more sensitive marker, by both flow cytometry and immunohistochemistry.

References:

Hartmann K, Escribano L, Grattan C, et al. Cutaneous manifestations in patients with mastocytosis: Consensus report of the European Competence Network on Mastocytosis; the American Academy of Allergy, Asthma & Immunology; and the European Academy of Allergology and Clinical Immunology. J Allergy Clin Immunol 2016; 137:35.
Valent P, Akin C, Escribano L, et al. Standards and standardization in mastocytosis: consensus statements on diagnostics, treatment recommendations and response criteria. Eur J Clin Invest 2007; 37:435.
Valent P, Horny HP, Escribano L, et al. Diagnostic criteria and classification of mastocytosis: a consensus proposal. Leuk Res 2001; 25:603.

Reprinted with permission from: Horny HP, Metcalfe DD, Akin C, et al. Mastocytosis. In: WHO classification of tumours of haematopoietic and lymphoid tissues, revised 4th edition, Swerdlow SH, Campo E, Harris NL, et al (Eds), IARC, Copyright © 2017.

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WHO diagnostic criteria for cutaneous and systemic mastocytosis