A Rash That Raised Red Flags: DRESS Syndrome Linked to Pegylated Interferon Alfa

Pegylated Interferon alfa (Besremi) is increasingly used to manage essential thrombocytosis and is known to cause hypersensitivity reactions, including urticaria, angioedema, and anaphylaxis. However, DRESS (Drug Reaction with Eosinophilia and Systemic Symptoms) has not been previously associated with this medication. We present what may be the first reported case of Besremi-induced DRESS.

The patient is a 55-year-old female with hypothyroidism, hypertension, and JAK2-positive thrombocytosis who was transitioned from Anagrelide to Besremi in March 2025 due to medication side effects. In June of 2025, she developed pruritus starting on her feet, which rapidly progressed into a diffuse erythematous, purpuric, burning rash with whole body involvement. She also developed facial and lower extremity edema, transaminitis (AST/ALT 46/73), and marked eosinophilia (1150 cells/µL). The delayed onset after drug initiation, systemic involvement, and eosinophilia raised a strong suspicion for DRESS. Oral steroids were ineffective, necessitating hospitalization and IV corticosteroids. Her symptoms improved gradually with inpatient treatment and a 10-day taper of dexamethasone. Complete resolution took over two weeks. Besremi was discontinued, and symptoms did not recur.

The timing of symptom onset, eosinophilia, organ involvement, and protracted clinical course support a diagnosis of DRESS over other differentials. There was no mucosal involvement or desquamation to suggest SJS/TEN, no pustules for AGEP, and no history of autoimmune disease. The patient's RegiSCAR score of 3 categorizes this as a "possible" case, but in the context of classic clinical features and exclusion of other causes, DRESS is the most likely diagnosis.

This case highlights the importance of recognizing DRESS as a potential, underreported adverse effect of Besremi.