The pathology workup for SM includes examination of PB and BM aspirate smears, BM biopsy, special stains to assess fibrosis (e.g. reticulin, trichrome, Gomori), and immunohistochemistry to quantify and phenotype MCs (tryptase, CD117, CD25, ±CD30) and assess AHN (eg, CD34, CD3, CD14, CD20, CD138, CD61, determined by histopathologic assessment). Because the MC infiltrate in ISM-AHN can be subtle, a tryptase immunohistochemical stain should be performed in select myeloid neoplasms such as AML with t(8;21) and MDS/MPNs such as CMML. As a corollary, in a series of >1500 patients with a WHO-based diagnosis of CMML, NGS identified KIT D816V in 6% of cases (T. Haferlach, Munich Leukemia Laboratory, Munich, Germany, personal communication), although KIT D816V positivity is not always accompanied by SM. Fibrosis is present in association with MC aggregates; fibrosis outside of MC aggregates suggests an AHN. Ancillary testing such as flow cytometry (preferred for assessment of CD2 on MC and phenotypic characterization of AHNs), karyotype, PDGFRA fluorescence in situ hybridization if eosinophilia, KIT D816V quantitative PCR (qPCR), and NGS myeloid gene panels are further discussed in National Comprehensive Cancer Network guidelines.⁶¹ 

Hyper, hypercellular; normo, normocellular; var, variable.

Additional somatic mutations commonly include SRSF2, ASXL1, RUNX1, CBL, JAK2, and EZH2. Only ∼25% of ISMs and SSMs have an additional somatic mutation present (±). In ASM and MCL, approximately one-half will have additional mutations (+), whereas in SM-AHN, additional mutations are common (++ or +++ depending on number of mutations).

Serum tryptase level is mildly increased in patients with ISM, but may be normal when there is low-level BM involvement.

Some dysplasias (or signs of myeloproliferation) may be seen in non-MC lineages, but criteria for diagnosis of an AHN are not met. Blood counts are normal or only slightly abnormal.

Hypercellularity in the BM away from MC aggregates may be seen.

Cytopenias are commonly seen in ASM and acute MCL, as C findings (hemoglobin <10 g/dL, absolute neutrophil count <1.0 × 10⁹/L, platelets <100 × 10⁹/L), but can also be seen secondary to an AHN. Chronic MCL, by definition, lacks C findings.

ASM in transformation denotes 5%-19% MCs on BM aspirate smear, which has a high rate of progression to MCL.

Most MCLs are aleukemic, as defined by <10% circulating MCs in PB; ≥10% circulating MCs in PB is leukemic MCL. Other subtypes include de novo (primary) or secondary MCL, acute or chronic MCL, and MCL with or without an AHN.

A mild increase in eosinophils (<1.5 × 10⁹/L) may be seen in MCL. Other cytoses, including eosinophilia ≥1.5 × 10⁹/L, suggest an AHN. Absolute monocytes >1.0 × 10⁹/L and >10% monocytes on PB differential raise concern for CMML. Either an absolute monocytosis or >10% monocytes, but not both, can suggest MDS/MPN unclassifiable.

Depending on the type of AHN, there may or may not be circulating blasts.

Discordantly high KIT D816V VAF in the PB when there is a low MC burden in the BM is a surrogate for multilineage involvement/AHN.