Table 5.

“B” findings and “C” findings in WHO and ICC diagnostic criteria.

	WHO	ICC
B findings	Infiltration grade (MC) in BM ≥30% in histology (IHC) and/or serum tryptase ≥200 ng/mLa) and/or KIT D816V VAF ≥10% in BM or PB leukocytes	High mast cell burden, >30% of BM cellularity by mast cell aggregates (assessed on BM biopsy) and serum tryptase >200 ng/mL
	Signs of myeloproliferation and/or myelodysplasia without a frank AHN; normal or mildly abnormal CBCs	Cytopenia (not meeting criteria for C findings) or -cytosis. Reactive causes are excluded, and criteria for other myeloid neoplasms are not met
	Organomegaly without dysfunction; hepatomegaly, splenomegaly or lymphadenopathy (>2 cm)	Hepatomegaly without impairment of liver function, or splenomegaly without features of hypersplenism including thrombocytopenia, and/or lymphadenopathy (>1 cm size) on palpation or imaging
C findings	BM dysfunction: HB <10 g/dL, and/or PLT <100 G/L, and/or neutrophils <1 G/L
	Hepatomegaly with liver dysfunction
	Splenomegaly with hypersplenism
	Large osterolysis (≥2 cm) with pathologic fracture±bone pain
	Malabsorption with weight loss due to GI MC infiltrates

^a)In the case of a known hereditary α tryptasemia (HαT), the basal serum tryptase level should be adjusted.

The diagnosis of variants of systemic mastocytosis require correlation with B and C findings. “B” findings represent burden of disease. “C”-findings represent SM induced organ damage.