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Background: Mastocytosis is a heterogeneous disease characterized by a clonal expansion of mast cells in various organs. The vast majority of patients suffer from signs and symptoms caused by mediator release from mast cells. Although the dis- ease burden is high, there is currently no specific and validated instrument to mea- sure and monitor signs and symptoms in patients with mastocytosis.

Objective: To develop and validate a disease-specific tool to measure and monitor the activity of signs and symptoms in patients with mastocytosis, the Mastocytosis Activity Score (MAS).

Methods: Nineteen potential MAS items were developed in a combined approach consisting of semi-structured patient interviews, expert input and literature research. Item selection was performed by impact analysis with 76 patients followed by a review for face validity. The resulting MAS was tested for validity, reliability and influence factors. In parallel, a US American English version of the MAS was devel- oped.

Results: A total of 68 mastocytosis patients took part in the MAS validation study. The final 9-item MAS was found to have a three-domain structure (“skin,” “gastroin- testinal tract” and “other”), a valid total score and an excellent test-retest reliability. Multiple regression analysis revealed that disease duration, age or gender is not a significant determinant of the MAS results.

Conclusions: The MAS is a disease-specific, valid and reliable patient-reported out- come measure for adult patients with cutaneous and indolent systemic mastocyto- sis. It may serve as a valuable tool to measure and monitor mastocytosis activity, both, in clinical trials and in routine care.