The EU-US Cooperative Group published updated recommendations about the standards of pathology in diagnosing systemic mastocytosis (SM) in a report in the Journal of Allergy and Clinical Immunology: In Practice. The updated recommendations were presented at the Year 2020 Working Conference held in Vienna, Austria.

Pathology continues to be key for the diagnosis and subtyping of SM and should be part of disease monitoring, especially in patients with advanced disease, according to the recommendations.

In the 2016 revision of the World Health Organization (WHO) classification of myeloid neoplasms, mastocytosis is no longer considered a subgroup of myeloproliferative neoplasm but rather a separate disease category. It comprises a group of diseases characterized by the proliferation and accumulation of excessive amounts of pathologic mast cells inside tissues. 

There are 3 main types of mastocytosis: cutaneous mastocytosis, SM, and mast cell sarcoma. SM is characterized by the presence of focal and/or diffuse infiltrates of neoplastic, spindle-shaped, hypogranulated mast cells in the bone marrow, liver, spleen, and gastrointestinal tract.

The WHO defines 5 diagnostic criteria for SM. These are compact mast cell aggregates, atypical mast cell morphology, activating KIT point mutations, and aberrant expression of CD25 and/or CD2 and/or CD30 in mast cells.

Read more about the diagnosis of SM

SM itself is classified into subgroups. These are bone marrow mastocytosis, indolent SM, smoldering SM, aggressive SM, SM with an associated hematologic neoplasm, and mast cell leukemia.

The final classification into one of these categories is of great importance in terms of prognosis for the patient. Certain morphological, clinical, radiological, and biochemical data should be considered when reaching a final diagnosis but this can be challenging for the pathologist and clinician. The recommendations presented in this article aim to facilitate this diagnosis.


Sotlar K, George TI, Kluin P, et al. Standards of pathology in the diagnosis of systemic mastocytosis: recommendations of the EU-US Cooperative Group. J Allergy Clin Immunol Pract. Published online June 18, 2022. doi:10.1016/j.jaip.2022.05.036