Abstract

Background: Mastocytosis often initially presents as a cutaneous rash leading to biopsy. A finding of 20 mast cells per high power field or at least one cluster of 15 mast cells has previously been recommended as a minor skin criterion for making the diagnosis of mastocytosis in the skin, which with further clinical information may be classified as cutaneous or systemic mastocytosis. However, the presence of 20 mast cells per high power field or clusters of 15 mast cells in urticaria and spongiotic dermatitis has not been previously evaluated. We suspect that mast cells may be present in these numbers in other inflammatory cutaneous conditions and that the criterion may be too sensitive and inadequately specific, leading to potential overdiagnosis of mastocytosis in the skin. Methods: Twenty-six cases with a diagnosis of urticaria or spongiotic dermatitis were identified from July 1, 2021 to January 12, 2022. Recuts were stained with mast cell tryptase and slides were reviewed. Additionally, 7 cases of mastocytosis were identified from 2015-2021 and these were reviewed. Results: 12 of 26 cases (46.1%) of urticaria and spongiotic dermatitis had at least 20 mast cells per high power field. However, only 3 of 26 cases (11.5%) of urticaria and spongiotic dermatitis had a cluster of 15 mast cells. 6 of 7 cases (85.7%) of mastocytosis had at least 20 mast cells per high power field; 4 of 7 cases (57.1%) of mastocytosis had a cluster of 15 mast cells. Conclusions: The finding of 20 mast cells per high power field is not specific to mastocytosis and can be seen in many cases of urticaria and spongiotic dermatitis. However, the finding of a cluster of 15 mast cells is more specific to mastocytosis and should be relied upon more heavily as a minor skin criterion to make the diagnosis in concert with the appropriate clinical presentation. Caution should be used when the only histologic finding leading to a diagnosis of mastocytosis is 20 mast cells per high power field.

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