What is the major criterion for systemic mastocytosis?

The major criterion for diagnosing systemic mastocytosis is the finding of multifocal dense infiltrates of mast cells in the bone marrow biopsy. This criterion is critical because it confirms that mast cells are not only present but are proliferating abnormally in the bone marrow, which is a reflection of the systemic nature of the disease.

While the presence of skin lesions, elevated serum tryptase levels, and systemic symptoms can all play a role in the clinical picture of systemic mastocytosis, they are not definitive for diagnosis. Skin lesions may indicate cutaneous mastocytosis or be present in systemic cases, but they alone do not establish the systemic involvement required for the diagnosis. Elevated serum tryptase levels can support the diagnosis but are not specific to systemic mastocytosis and can occur in other conditions. Similarly, systemic symptoms may suggest a more serious condition, but they do not confirm the diagnosis without confirming infiltrates in the bone marrow. Thus, the identification of a bone marrow biopsy showing multifocal dense infiltrates of mast cells stands as the most definitive criterion in determining systemic mastocytosis.