Bone marrow mast cell burden and serum tryptase level as markers of response in patients with systemic mastocytosis.

Two important response criteria in systemic mastocytosis (SM) are the elimination or reduction in percentage of bone marrow mast cells (MCs) and the reduction of serum tryptase levels. We investigated the accuracy of a single time point reduction of bone marrow MCs and serum tryptase level as response criteria in 50 patients with SM with available serial assessments. Bone marrow MC percentage varied significantly, with an average coefficient of variation (CV) of 65% (range, 6-173%) and 44% of patients having a CV higher than the average. The average CV for serum tryptase level was 19% (range, 0-96%), with 36% of patients having a CV higher than average. These wide variations in bone marrow MC burden and serum tryptase level were independent of the administration of SM-directed therapy, type of therapy or disease subtype. Furthermore, the achievement of a single time point therapy-induced bone marrow complete response (no histological evidence of malignant bone marrow MCs) did not correlate with tryptase level or symptomatic improvement. In conclusion, the value of single measurements of bone marrow MC percentage and serum tryptase level as response criteria in SM is not supported by clinical data. Incorporation of an assessment of the degree in reduction of MCs and tryptase, and assessment of response durability, would make response criteria more clinically meaningful.