Systemic mastocytosis (SM) is frequently associated with eosinophilia. To examine its prevalence and clinical impact in all WHO classification-based subcategories, we analyzed eosinophil counts in 2350 mastocytosis patients using the dataset of the European Competence Network on Mastocytosis. Ninety percent of patients had normal eosinophil counts, 6.8% mild eosinophilia (0.5-1.5 × 109/l), and 3.1% hypereosinophilia (HE; >1.5 × 109/l). Eosinophilia/HE were mainly present in patients with advanced SM (17%/19%), and only rarely recorded in patients with indolent and smoldering SM (5%/1%), and some patients with cutaneous mastocytosis. The eosinophil count correlated with organomegaly, dysmyelopoiesis, and the WHO classification, but not with mediator-related symptoms or allergy. Eosinophilia at diagnosis had a strong prognostic impact (p < 0.0001) on overall survival (OS) and progression-free survival (PFS), with a 10-year OS of 19% for patients with HE, 70% for those with mild eosinophilia, and 88% for patients with normal eosinophil counts. In 89% of patients with follow-up data (n = 1430, censored at start of cytoreductive therapy), eosinophils remained stable. In those with changing eosinophil counts (increase/decrease or mixed pattern), OS and PFS were inferior compared with patients with stable eosinophil counts. In conclusion, eosinophilia and HE are more prevalent in advanced SM and are predictors of a worse outcome.

3 Medizinische Klinik Universitätsmedizin Mannheim Universität Heidelberg Mannheim Germany

Aiichi Medical University Nagakute Japan

Allergy Unit Verona University Hospital Verona Italy

Department Immunol Genetics and Pathology Uppsala University Hospital Uppsala University Uppsala Sweden

Department of Allergology Medical University of Gdansk Gdańsk Poland

Department of Dermatology and Allergy Biederstein School of Medicine Technical University of Munich Munich Germany

Department of Dermatology and Allergy Justus Liebig University Giessen Giessen Germany

Department of Dermatology and Venereology Medical University of Graz Graz Austria

Department of Dermatology and Venerology Kepler University Hospital Johannes Kepler University Linz Austria

Department of Dermatology Medical Center University of Freiburg Freiburg Germany

Department of Dermatology University Hospitals Leuven Leuven Belgium

Department of Dermatology University of Cologne Cologne Germany

Department of Dermatology Venereology and Allergology Medical University of Gdansk Gdańsk Poland

Department of Hematology and Oncology Medical Center University of Schleswig Holstein Campus Lübeck Lübeck Germany

Department of Hematology Gustave Roussy Cancer Center Villejuif France

Department of Hematology Oncology and Stem Cell Transplantation Medical Center Faculty of Medicine University of Freiburg Freiburg Germany

Department of Hematology Semmelweis University Budapest Hungary

Department of Hematology University Medical Center Groningen University of Groningen Groningen The Netherlands

Department of Internal Medicine 1 Division of Hematology and Hemostaseology Vienna Austria

Department of Internal Medicine Section Allergology University Medical Center Groningen University of Groningen Groningen The Netherlands

Department of Molecular Medicine and Department of Hematology Oncology University of Pavia and Fondazione IRCCS Policlinico San Matteo Pavia Italy

Department of Oncology Haematology Haemostaseology and Stem Cell Transplantation University Hospital RWTH Aachen Aachen Germany

Division of Allergy and Clinical Immunology University of Salerno Salerno Italy

Division of Allergy Department of Dermatology University of Basel Basel Switzerland

Division of Hematology Department of Medicine Stanford University School of Medicine Stanford CA USA

Division of Hematology Istanbul Medical School University of Istanbul Istanbul Turkey

Faculty of Medicine and Health Sciences Department of Immunology Allergology Rheumatology University of Antwerp and Antwerp University Hospital Antwerpen Belgium

Faculty of Medicine University of Freiburg Freiburg Germany

French Reference Center for Mastocytosis Hôpital Necker Assistance Publique Hôpitaux de Paris Imagine Institute University Paris Descartes Paris France

German Cancer Consortium Partner Site Freiburg Freiburg Germany

Laboratory of Hematology Pitié Salpêtrière Hospital Paris France

Ludwig Boltzmann Cluster Oncology Medical University of Vienna Vienna Austria

Necker Pasteur Center for Infectious Diseases and Tropical Medicine Paris Descartes University Paris France

Pediatric Dermatology Unit Department of Medicine University of Padova Padova Italy

Section of Hematology Department of Medicine Verona University Hospital Verona Italy

University Clinic for Hematology and Oncology Johannes Kepler University Linz Austria

University Hospital Brno Brno Czech Republic

University Hospital of Leipzig Leipzig Germany

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