Clinical outcomes and nephrotoxicity of colistin loading dose for treatment of extensively drug-resistant Acinetobacter baumannii in cancer patients Wasan Katip,1 Suriyon Uitrakul,2 Peninnah Oberdorfer3 1Department of Pharmaceutical Care, Faculty of Pharmacy, Chiang Mai University, Chiang Mai, Thailand; 2Northern Institute for Cancer Research, Newcastle University, Newcastle upon Tyne, UK; 3Division of Infectious Diseases, Department of Pediatrics, Faculty of Medicine, Chiang Mai University, Chiang Mai, Thailand Background: Colistin is a last-line defense therapy against extensively drug-resistant Acinetobacter baumannii (XDR-AB). Despite a loading dose of colistin being applied in many clinical practices, studies evaluating the effect of the loading dose of colistin in cancer patients remain limited.Patients and methods: A retrospective cohort study of cancer patients who received either a loading or non-loading dose of colistin for treatment of XDR-AB was conducted. For each group, the clinical response, bacteriological eradication and serum creatinine were recorded. Logistic regression was applied to evaluate the effects of therapy on each of the three aforementioned outcomes.Results: One hundred and two patients diagnosed with XDR-AB infections between January 2012 and December 2015 were recruited. Only 75 patients were given a loading dose of colistin. There was no significant clinical and microbiological response in patients in the loading dose group or patients in the non-loading dose group. However, 38 (50.67%) patients in the loading dose group and 6 (22.22%) patients in the non-loading dose group developed nephrotoxicity according to the RIFLE criteria (p