The major criterion is The presence of multifocal, dense aggregates of > 15 mast cells in bone marrow (preferred) or other extracutaneous organs (except the GI tract, lymph nodes, liver, or spleen) The 4 minor criteria are Atypical morphology or spindle shapes in > 25% of the mast cells in bone marrow biopsy sections or aspirate A kit mutation at codon 816 (commonly Asp816Val) in bone marrow, peripheral blood, or other tissue Bone marrow or other extracutaneous mast cells expressing the surface markers CD2, CD25, or both Baseline serum tryptase levels > 20 ng / mL (> 20 mcg / L); values > 11.4 ng / mL (11.4 mcg / L) are considered elevated in most diagnostic laboratories) The baseline level of tryptase is elevated in systemic mastocytosis but is typically normal in cutaneous mastocytosis and mast cell activation syndrome.