Turkiye Klinikleri J Case Rep. 2022;30(1):40-3 40 Eosinophilia is defined as a peripheral eosi- nophil count (PEC) greater than or equal to 500 cells/μL. Besides; hypereosinophilia, the eosinophil count is over 1,500 cells/μL. 1 It may be observed in various diseases such as infections, allergies, neo- plasms, and primary hematologic malignancies. Also, eosinophilia affects 25% of patients on parenteral an- tibiotics. Drug-induced eosinophilia can be seen as asymptomatic; also, it can be seen in different clini- cal pictures ranging from mild to severe hypersensi- tivity reactions (HSRs). Asymptomatic eosinophilia is mostly associated with penicillins, cephalosporins, and fluoroquinolones, among antimicrobials. 2 Colistin (polymyxin E) was discovered in 1949 as a polycationic peptide antimicrobial, and it is ad- ministered as a prodrug form, namely colistimethate- sodium (CMS). Colistin is often the last optional drug for nosocomial infections caused by multidrug-resis- tant (MDR) gram-negative microorganisms, espe- cially carbapenemase-producing Enterobacter spp., Pseudomonas aeruginosa, and Acinetobacter bau- manni. 3 The common adverse effects of colistin are dizziness, numbness, tingling, prickling and burning sensation. Also, colistin may induce renal and neuro- logical adverse effects. On the other hand, colistin may cause mild to severe HSRs and eosinophilia, ac- cording to previous reports. 4 Here we presented a case with early-onset of eosinophilia due to parenteral colistin therapy. CASE REPORT An 84-year-old female patient was hospitalized with a loss of consciousness in the neurologic intensive care unit (ICU). Her past medical history included hypertension, tremor, allergic asthma, coronary ar- tery disease, and Type 2 diabetes mellitus. She was Asymptomatic Eosinophilia Induced by Systemic Colistin Therapy: A Brief Case Report İzgi BAYRAKTAR a , Emre KARA a , Meliha Çağla SÖNMEZER b , Kutay DEMİRKAN a , Mehmet Akif TOPÇUOĞLU c a Department of Clinical Pharmacy, Hacettepe University Faculty of Pharmacy, Ankara, Türkiye b Department of Infectious Diseases and Clinical Microbiology, Hacettepe University Faculty of Medicine, Ankara, Türkiye c Department of Neurology, Hacettepe University Faculty of Medicine, Ankara, Türkiye ABSTRACT Colistin is a polymyxin group antibiotic, and colistimethate-sodium is the prodrug form of colistin and is inactive. Colistin is often the last optional drug for nosocomial infections caused by multidrug-resistant gram-negative microorganisms. We present a case of asymptomatic eosinophilia due to systemic colistin therapy. The increasing trend of absolute eosinophil counts appeared with recurrent colistin therapy and de- creased when colistin was discontinued. The Naranjo Adverse Drug Reaction Probability Scale score was 7, and this score was classified as “probable” for this case. Clinicians should consider this drug reaction in a patient with eosinophilia and avoid unnecessary laboratory tests and treatments. Keywords: Colistin; colistimethate sodium; eosinophilia; drug reaction DOI: 10.5336/caserep.2021-85477 CASE REPORT Correspondence: İzgi BAYRAKTAR Department of Clinical Pharmacy, Hacettepe University Faculty of Pharmacy, Ankara, Türkiye E-mail: izgibayraktar@gmail.com Peer review under responsibility of Turkiye Klinikleri Journal of Case Reports. Received: 14 Jul 2021 Received in revised form: 29 Sep 2021 Accepted: 29 Sep 2021 Available online: 07 Oct 2021 2147-9291 / Copyright © 2022 by Türkiye Klinikleri. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Turkiye Klinikleri Journal of Internal Medicine Türkiye Klinikleri Journal of Case Reports