Seasonal Variations in the Risk of Outpatient Acute Kidney Injury in Patients with Chronic Kidney Disease

<b>Background/Objectives</b>: Acute kidney injury (AKI) frequently occurs in the outpatient setting and is associated with adverse renal and survival outcomes. However, there is no established definition of outpatient AKI, and the risk factors, especially seasonal variation, remain limited. This study aimed to investigate seasonal variation in the risk of outpatient AKI. <b>Methods</b>: This retrospective observational study used routinely collected clinical laboratory data from a single hospital in Japan between 2007 and 2022. Outpatient AKI was defined as ≥35% relative decline in estimated glomerular filtration rate (eGFR) compared with a preceding outpatient measurement obtained within 14-90 days. Monthly and seasonal variations in outpatient AKI risk in patients with chronic kidney disease (CKD) were evaluated using logistic regression models. Subgroup analyses were performed according to AKI stage, age group, and CKD stage. <b>Results</b>: A total of 203,853 outpatient records were analyzed. The incidence of outpatient AKI was highest in August and lowest in November. Analyses demonstrated significantly increased odds ratios of outpatient AKI in January, February, July, and August. Seasonally, the risk was significantly higher during the summer. Stage-specific analyses showed that AKI stage 1 was more frequent in the summer, whereas AKI stage 2 tended to increase during the winter. <b>Conclusions</b>: Outpatient AKI exhibits distinct seasonal patterns, with increased risk during both summer and winter and differential associations according to AKI severity and baseline kidney function. Recognition of these patterns may help identify vulnerable populations and inform targeted preventive strategies for outpatient AKI.