Hypereosinophilic syndrome with elevated serum tryptase versus systemic mast cell disease associated with eosinophilia: 2 distinct entities?

Abstract
Clinical presentation in adults with SMCD is markedly heterogeneous,2 and making or refuting the diagnosis requires a careful morphologic analysis of the bone marrow. In general, SMCD is characterized by focal, dense aggregates of dysplastic MCs. However, the bone marrow MC infiltration pattern in aggressive SMCD, including SMCD-eos, can be diffuse, and whether one appreciates a “dense” or “loose” scattering of MCs in this setting is open to subjective bias.3 In our experience, either dense or loose aggregates of dysplastic MCs are seen in both FIP1L1-PDGFRA+ and c-kit D816V+ SMCD-eos. This was illustrated in a recent report of 5 patients with SMCD-eos in which all 3 patients who carried the FIP1L1-PDGFRA fusion had pathognomonic MC aggregates in the bone marrow in a pattern that was not different from 1 of the patients with the c-kit D816V mutation.4

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