In Systemic Mastocytosis multifocal clustering of  mast cells (at least 15 MCs/cluster) can be found within the bone marrow chunks. Since mast cells in most cases are only slightly elevated and ranges from 0.1% to a maximum of about 2% of cells, and often show abnormal morphology (immature forms, spindling, decentralized oval nuclei or bi- or poly-lobed nuclei, hypogranulated cytoplasm).

Diagnostic criteria