An updated international consensus classification (ICC) of the multiple forms of mastocytosis introduces refinements to the diagnostic criteria for systemic mastocytosis (SM) and for diagnosing mast cell leukemia (MCL), according to a review by a group of experts.

“The presence of the major diagnostic criterion is now adequate to diagnose SM without additional need of a minor criterion,” the authors explained in Virchows Archives. “However, the major criterion now includes the confirmation of the [mast cell (MC)] nature by immunohistochemistry.”

SM presents as indolent SM (ISM), smoldering SM (SSM), SM with an associated hematologic neoplasm (SM-AHN), aggressive SM, or MCL.

The major criterion for sufficiently diagnosing SM is confirmed by the use of tryptase and/or CD117 immunoreactivity to ensure proper identification of MCs: multifocal dense aggregates of mast cells (≥15 mast cells per aggregate) seen in bone marrow biopsy specimens and/or in sections of other extracutaneous organs.

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In the absence of that criterion, at least 3 of the 4 minor criteria should be present to reach a diagnosis of SM. These criteria include: (1) more than 25% of biopsied mast cells have an atypical morphology; (2) mast cells express CD25, CD2, or (newly added in 2022) CD30 in addition to mast cell markers; (3) KIT D816Vmutation or (newly added in 2022) other activating KIT mutations in bone marrow, peripheral blood or other organs; and (4) persistently elevated serum tryptase of more than 20 ng/mL.

Furthermore, in the new classification, the “B” findings that distinguish SSM from ISM have been simplified. The first criterion of a high MC burden and high serum tryptase is unchanged; the second criterion is now “cytopenia (not meeting criteria for C-findings) or -cytosis”. The third criterion specifies the characteristics of splenomegaly further as palpable and without features of hypersplenism such as thrombocytopenia.

Reference

Leguit RJ, Wang SA, George TI, et al. The international consensus classification of mastocytosis and related entities.Virchows Arch. Published online October 10, 2022. doi: 10.1007/s00428-022-03423-3