A comparison of mast cells in skin biopsies of cutaneous mastocytosis with other inflammatory dermatoses: A study of 33 cases

Background Histopathologic criteria for diagnosis of cutaneous mastocytosis include 20 mast cells per high‐power field or clusters of 15 mast cells. We aimed to determine the specificity of these criteria for cutaneous mastocytosis in comparison with inflammatory disorders of mast cell activation. Methods Twenty‐six cases of spongiotic dermatitis or urticaria were identified from 2021 to 2022. Recuts were stained with mast cell tryptase and slides were reviewed for the presence of 20 mast cells per high‐power field and for clusters of 15 mast cells. In addition, seven cases of mastocytosis were reviewed for the same criteria. Results Twelve of 26 cases (46.1%) of spongiotic dermatitis/urticaria had at least 20 mast cells per high‐power field. Three of 26 cases (11.5%) of spongiotic dermatitis/urticaria had a cluster of 15 mast cells. Six of seven cases (85.7%) of mastocytosis had at least 20 mast cells per high‐power field; four of seven cases (57.1%) of mastocytosis had a cluster of 15 mast cells. Conclusions In our study, the finding of 20 mast cells per high‐power field was nonspecific as a single criterion for cutaneous mastocytosis. The finding of clusters of 15 mast cells was more specific but not sensitive.