Table 2.

Summary of the characteristic features of classical hairy cell leukemia and hairy cell leukemia variant

Characteristic Features	IGHV4-34- cHCL	IGHV4-34+ cHCL	vHCL
Laboratory parameters
Pancytopenia	Present	Absent	Absent
Monocytopenia	Present	Absent	Absent
Absolute neutrophil count	Decreased	Normal	Normal
Leukocytosis	Absent	Present	Present
Soluble IL2 receptor (plasma)	Elevated	Unknown	Normal
Bone marrow aspiration	Dry tap	Easily aspirated	Easily aspirated
Immunophenotypic profile
CD19	++	++	++
CD20	++	++	++
CD22	++	++	++
FMC7	+	+	+
CD11c	++	++	++
CD103	+	+	+
CD25	+	+	––
CD123	+	+	––
CD200	+	+	––
Annexin A1	+	+	––
Histological features
Nucleus (Wright–Giemsa)	Oval/kidney-shaped	Oval/kidney-shaped	Round/oval
Nucleolus (Wright–Giemsa)	Absent	Absent	Prominent
Cytoplasm (Wright–Giemsa)	Abundant, pale	Abundant, pale	Abundant, pale
Cellular surface	Thin, often long circumferential projections	Thin, often long circumferential projections	Thin, often long circumferential projections
Infiltration pattern (bone marrow)	Diffuse and/or interstitial	Diffuse and/or interstitial	Diffuse, intrasinusoidal, and/or interstitial
Infiltration pattern (spleen)	Red pulp with effacement of white pulp	Red pulp with effacement of white pulp	Red pulp
TRAP immunocytochemistry	+	+	––
Annexin A1 immunohistochemistry	+	+	––
BRAFV600E immunohistochemistry	+	––	––
Molecular features
BRAFV600E mutations	Present (∼97%)	Absent	Absent
MAP2K1 alterations	Absent	Present	Present (∼50%)
IGHV somatic hypermutation	Frequent	Absent	Rare
IGHV4-34usage	Absent	Present	Frequent
CDKN1B (p27) co-occurring mutations	Present	Unknown	Absent
KLF2 co-occurring mutations	Present	Unknown	Absent
NOTCH1 co-occurring mutations	Rare	Unknown	Absent
NOTCH2 co-occurring mutations	Rare	Unknown	Absent
BCOR co-occurring mutations	Rare	Unknown	Absent
KDM6A co-occurring mutations	Rare	Unknown	Frequent
CREBBP co-occurring mutations	Rare	Unknown	Frequent
CCND3 co-occurring mutations	Absent	Unknown	Present
U2AF1 co-occurring mutations	Absent	Unknown	Present

cHCL, Classical hairy cell leukemia; vHCL, hairy cell leukemia variant; IL2, interleukin 2; CD, cluster of differentiation; IGHV, immunoglobulin heavy chain variable; +, low expression; ++, high expression; –, not detected.