TABLE III.

Classification of syndromes and conditions accompanied by HE

Variant	Typical findings
HES*
Idiopathic HES	No underlying cause of HE, no evidence of a reactive or neoplastic condition/disorder underlying HE and end-organ damage attributable to HE
Primary (neoplastic) HES (HESN)
Secondary (reactive) HES (HESR)	Underlying stem cell, myeloid, or eosinophilic neoplasm classified according to WHO guidelines and end-organ damage attributable to HE, and eosinophils are considered (or shown) neoplastic (clonal) cells.†
	Underlying condition/disease in which eosinophils are considered nonclonal cells; HE is considered cytokine driven, and end-organ damage is attributable to HE.
	Subvariant: lymphoid variant HES (clonal T cells identified as the only potential cause)‡
Other conditions and syndromes
Specific syndromes accompanied by HE	Specific syndromes in which the effect of eosinophilia remains unclear but the clinical presentation is distinct and accompanied by HE; specific syndromes are listed in Table E3.
Other conditions accompanied by HE	Mostly organ-restricted conditions in which the effect of eosinophilia remains unclear; an overview of organ-restricted pathologies accompanied by HE is shown in Table E4, and that of skin disorders accompanied by eosinophilia is shown in Table E5.

^*HES is defined as blood HE with (plus) end-organ damage attributable to tissue HE.

^†Clonality of eosinophils is often difficult to demonstrate or is not examined. However, if a myeloid or stem cell neoplasm known to present typically with clonal HE is present or a typical molecular defect is demonstrable (eg, PDGFR or FGFR mutations or BCR/ABL1), eosinophilia should be considered clonal.

^‡The lymphoid variant of HES is regarded as a special form of secondary HES by several experts, although its exact nature and pathogenesis remain controversial.