The World Health Organization (WHO) has given criteria for making the diagnosis of systemic mastocytosis.


Major criteria:

(1) biopsy of bone marrow or extracutaneous tissue shows multifocal, dense infiltrates of mast cells (cellular aggregates with >= 15 mast cells, confirmed by tryptase immuno-histochemistry or special stains)


Minor criteria:

(1) biopsy of bone marrow or extracutaneous tissue show > 25% mast cells spindle-shaped or atypical OR bone marrow aspirate show > 25% mast cells immature or atypical

(2) detection of KIT point mutation at codon 816 in bone marrow or extracutaneous tissue

(3) mast cells in bone marrow, blood or extracutaneous tissue that coexpresses

(3a) CD117 and CD2 AND/OR

(3b) CD117 and CD25

(4) serum total tryptase persistently > 20 ng/mL AND no clonal myeloid disorder



• An elevation in serum tryptase may occur in clonal myeloid disorder (AML, myeloproliferative disorder, myelodysplastic syndrome) without evidence of mast cell proliferation or mastocytosis.

• CD117 marks for c-kit.

• Atypical mast cells may be Type I or Type II.

• Atypical mast cells Type I have 2 or 3 of the following findings: (a) prominent surface projections or be spindle-shaped, (b) oval nucleus with or without an eccentric location, (c) hypogranulated cytoplasm with focal accumulations of granules with or without granule fusions but no signs of degranulation.

• Atypical mast cells Type II have (a) a bilobar or polylobar nucleus. (b) nuclear-to-cytoplasmic ratio that may be high (immature) or low (mature), (c) nuclear chromatin that may be fine (immature) or condensed (mature), (d) variable presence of nucleoli, and (e) hypogranulated cytoplasm without signs of degranulation.


Criteria for diagnosis:

(1) 1 major and >= 1 minor criteria

(2) 0 major and >= 3 minor criteria


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