Table 4. Investigations recommended in patients with HB^a .

Primary, non-invasive Case history (ask specifically for allergies and mutagenic events) Spleen size and lymph node status Complete blood picture (repeated to confirm HB) Serum chemistry, including serum tryptase level Allergy diagnostics, including total IgE BCR-ABL1 by RT-PCR and/or FISH JAK2 V617F by RT-PCR FIP1L1-PDGFRA by RT-PCR and/or FISH (if also eosinophilia is present) JAK2 exon 12-, MPL- and CALR mutation (if MPN-like features are present, but no BCR-ABL1 and no JAK2 V617F is found)

Secondary (when no BCR-ABL1 is found and no other underlying cause of HB is identified after primary testing) Bone marrow histology and immunohistochemistry Investigation of bone marrow smears (morphology) Chromosome analysis and extended FISH panel (for MDS and/or MPN) if indicatedb Molecular studies, including NGS (if available) and T-cell receptor rearrangement Extensive allergy diagnostics Inflammation markers

Abbreviations: CALR, calreticulin; FISH, fluorescence in situ hybridization; IgE, immunoglobulin E; RT-PCR, reverse transcriptase polymerase chain reaction; MDS, myelodysplastic syndromes; MPN, myeloproloferative neoplasms; NGS, next-generation sequencing studies.

^aHB is defined as persistent basophilia with a basophil count of > 1000 per μl of blood.

^bAdditional FISH studies are recommended when conventional karyotyping is inconclusive or did not work (no growth of cells) and molecular studies did not reveal a specific aberration. Depending on the clinical presentation (for example, signs of MDS) and presence of (additional) laboratory abnormalities such as eosinophilia, the FISH panel should cover MDS-related molecular aberrations (standard MDS panel according to local institutional guidelines) and MPN-related lesions, including FIP1L1-PDGFRA (CHIC2 del), PDGFRB rearrangements (5q33), FGFR1 rearrangements (8p11), MYB-GATA1, monosomy 7 and 7q del (cen 7/7q31), trisomy 8 (cen 8), trisomy 9 (cen 9) and 20q deletion (20q21).