Aim Elderly patients undergoing abdominal surgery have a high risk of developing postoperative delirium. This study aimed to explore risk factors for postoperative delirium by focusing on preoperative hematologic markers, and to develop a model that would be useful in predicting the occurrence of postoperative delirium. Methods This retrospective study included patients aged ≥65 years who underwent elective abdominal surgery under general anesthesia from January 2017 to May 2019. Patients' demographics, hematologic data and intraoperative data were recorded. Neutrophil‐to‐lymphocyte ratio, platelet‐to‐lymphocyte ratio, platelet‐to‐white blood cell ratio and prognostic nutritional index were calculated. We evaluated risk factors of postoperative delirium assessed using a chart‐based method for identification of delirium and developed a predictive model using preoperative and intraoperative data. Results Of 931 eligible patients, 833 with a mean age of 73.9 years were included in the analysis. The rate of incidence of postoperative delirium was 7.3%. Age, duration of surgery and preoperative platelet‐to‐lymphocyte ratio or platelet‐to‐white blood cell ratio were statistically significant risk factors. The predictive model, which included age, duration of surgery and platelet‐to‐lymphocyte ratio, had the largest area under the curve of 0.77. Conclusions Age, duration of surgery and preoperative platelet‐to‐white blood cell ratio or platelet‐to‐lymphocyte ratio are associated with the development of postoperative delirium. A predictive model, which includes age, duration of surgery and preoperative platelet‐to‐lymphocyte ratio, is useful in predicting the development of postoperative delirium after elective abdominal surgery. Geriatr Gerontol Int 2020; 20: 1208–1212.

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