Quantifying Mast Cell and Eosinophil Cellular Density in Skin Biopsy Tissue From Adults With Maculopapular Cutaneous Mastocytosis as Compared With Urticaria and Normal Skin: A Retrospective Histopathologic Study

Background: Maculopapular cutaneous mastocytosis (MPCM) is a rare disorder characterized by a pathologic accumulation of mast cells in the skin, which may or may not be accompanied by systemic mastocytosis. Diagnosis of MPCM on skin biopsy can be challenging because the findings may be subtle. Although mast cell density in MPCM has been reported, data informing a proposed cutoff for diagnosis and diagnostic criteria are limited. Methods: We identified adult patients diagnosed with MPCM and urticarial tissue reaction/chronic urticaria on skin biopsy and compared the mast cell and eosinophil counts per 1 mm 2 in 10 cases each of MPCM, chronic urticaria, and normal skin from routine biopsies. All slides were stained with CD117, and CD117-positive mast cells were counted per 1 mm 2 using digital microscopy. Eosinophils were counted on hematoxylin and eosin-stained slides per 1 mm 2 using digital microscopy. Results: The median number of mast cells per 1 mm 2 was significantly higher in MPCM than in cases of urticaria and normal skin/control tissue (177.3 vs. 26.8 vs. 47.8 mast cell per mm 2 , respectively; P ≤ 0.001). The calculated “cut point” for mastocytosis versus chronic urticaria and normal skin was 66 mast cells per 1 mm 2 , whereas the value for controls versus urticaria was 37 mast cells per 1 mm 2 . Eosinophils had similar density in MPCM and urticaria, and their presence was significant in the differentiation of MPCM and urticaria from normal tissue. Conclusions: This study adds to the literature by providing objective mast cell density data to distinguish challenging cases of cutaneous mastocytosis from urticarial reactions and normal skin. Future studies could explore the development of computer-aided estimations of cellular density with more extensive comparison with other inflammatory conditions to translate our findings more readily into clinical practice.