An 83-year-old man with an erythematous rash and sustained eosinophilia (>1.5 with peak 12.2 × 10⁹/L; panel A, peripheral blood, Wright-Giemsa stain, original magnification ×50) presented to our hospital. With kidney injury present (creatinine level of 2.14 mg/dL), differential diagnoses of vasculitis vs DRESS (drug rash with eosinophilia and systemic symptoms) were considered. Computed tomography ruled out solid tumors. A skin biopsy showed superficial dermatitis. He tested negative for BCR-ABL, PDGFR-α/β, and FGFR-1 rearrangements, JAK2/C kit (D816V) mutations, and strongyloides serology, showing normal serum tryptase and immunoglobulin G4. Multiplex PCR (IdentiClone TCR β Gene Clonality Assay protocol targeting V, D, and J region of the amplified genomic DNA) showed a T-cell clonal process. Eosinophilic hyperplasia was seen (black arrow in panel B, bone marrow aspirate, May-Grunwald-Giemsa stain, original magnification ×50; and panels C-D, biopsy, hematoxylin and eosin stain, original magnification ×10 [C] and ×50 [D]). CD20 (not shown) and CD3 stains showed reactive T and B cells (panel E, original magnification ×10). Specimen flow cytometry showed an abnormal T-cell population (panel F, teal-colored events) expressing CD3 (−/dim), CD2⁺, CD4⁺, and CD5⁺, being negative for CD7 or CD8.

Lymphocytic variant hypereosinophilic syndrome is an underrecognized cause of reactive eosinophilia. CD4⁺/CD3⁻, CD4⁺/CD7⁻ variants found here and CD3⁺/CD4⁻/CD8⁻ are the 3 major subtypes. Steroid-sparing treatments include interferon-α, cyclosporine, anti-interleukin-5–directed therapy (mepolizumab and benralizumab), and tofactinib.