A Case of Serum Sickness-Like Reaction in an Adult Treated with Omalizumab.

Omalizumab has been safely used to treat asthma and urticaria. We report a case of serum sickness-like reaction in a patient treated with omalizumab for chronic idiopathic urticaria/angioedema. An adult female experienced episodic urticaria/angioedema without repeatable trigger, ultimately receiving diagnosis of chronic idiopathic urticaria/angioedema. After initial treatment, attempts with escalating cetirizine and montelukast doses were unsuccessful due to sedation; she began treatment with subcutaneous omalizumab 150 mg monthly. Urticaria frequency partially improved after two injections; therefore, the dose was increased to 300 mg after four treatments. Several days after first 300 mg dose, she reported abdominal cramping, fatigue, fever, lymphadenopathy, and arthralgia. Aside from mild thrombocytosis, inflammatory markers were unremarkable, as were evaluations for infection, autoimmunity, and malignancy. Omalizumab was held with eventual improvement in symptoms, which did not return after discontinuation. Omalizumab is a helpful medication in treating atopic conditions, with at least theoretical risk of immune complex formation and tissue deposition causing serum sickness-like reaction. Although early publications showed very low adverse event rates, there have now been reports of serum sickness-like reactions in children and adults treated for asthma and urticaria. Determining true incidence is difficult, given rarity and non-specific nature. Previous reports described symptoms with initiation of medication, reproducible after reintroduction. While it remains to be determined which factors increase risk for serum sickness-like reaction to omalizumab, our report of an urticaria patient who exhibited symptoms with increasing dose contributes insight into the discussion regarding this adverse effect of an otherwise well-tolerated and important medication. © Association of Military Surgeons of the United States 2019. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.