Cutaneous mastocytosis in adults with a serum tryptase level < 20 ng mL–1: why we should investigate further

Dear Editor, Mastocytosis is a rare disease characterized by the accumulation/proliferation of abnormal mast cells (MCs).1 Mastocytosis is categorized into isolated cutaneous mastocytosis (CM), where only the skin is infiltrated by abnormal MC, and systemic mastocytosis (SM), where at least one internal organ is involved.2 SM is diagnosed according to the criteria defined by the World Health Organization (WHO).3 The major criterion consists of the presence of multifocal dense infiltrates of MC in a bone marrow (BM) biopsy or extracutaneous tissues. The minor criteria comprise the following: the presence of > 25% of MC with abnormal cytology out of all MCs in a BM biopsy or in other nonskin tissues; abnormal expression of CD2 and/or CD25 on MCs in an immunohistochemistry study and/or by flow cytometry analysis; the presence of the KIT codon D816V mutation in BM aspirate and a serum tryptase level (STL) > 20 ng mL–1. SM is diagnosed if the major criterion and at least one minor criterion or three minor criteria are present. For the diagnosis of SM, performing a BM aspirate and/or a BM biopsy is mandatory. For a patient presenting with CM, the first question for the physician is whether the disease is limited to the skin or does it involve internal organs? Answering this question is of relevance as specific treatments are available or under investigation in patients with SM – especially those with disabling MC activation symptoms or advanced SM. Screening basal STL in patients with CM is considered to be a cost-effective first-line tool to distinguish possible SM from isolated CM.