Tryptase and histamine metabolites as diagnostic indicators of indolent systemic mastocytosis without skin lesions

Background Risk indicators of indolent systemic mastocytosis (ISM) in adults with clinical suspicion of ISM without accompanying skin lesions [urticaria pigmentosa (UP)] are lacking. This study aimed at creating a decision tree using clinical characteristics, serum tryptase, and the urinary histamine metabolites methylimidazole acetic acid (MIMA) and methylhistamine (MH) to select patients for bone marrow investigations to diagnose ISM. Methods Retrospective data analysis of all adults, in whom bone marrow investigations were performed to diagnose ISM, was carried out. Results In total, 142 patients were included. SM was absent in all 44 patients with tryptase 20 μg/l. Above 43 μg/l, all patients had ISM (n = 11). Male gender, insect venom anaphylaxis as presenting symptom, tryptase, MIMA, and MH were independent ISM predictors. If tryptase was ≥10 μg/l, the diagnostic accuracy of MIMA and MH was high (areas under the ROC curve 0.92). Conclusions In suspected patients without UP, the ISM risk is very low (if present at all) if tryptase is