Seasonality of acute kidney injury incidence in Japanese outpatients.

Acute kidney injury (AKI) is a clinically important condition associated with adverse outcomes. While community-acquired AKI is common, its seasonal patterns remain poorly understood, particularly in outpatient settings. This study aimed to examine the seasonality and community-acquired AKI in Japan. Of 4,401,387 individuals with at least 1 month of data in the Japanese health insurance claims database between August 2016 and July 2018, 4,282,805 were included in the analysis as outpatients. The monthly incidence of community-acquired AKI was evaluated using the incidence rate ratio (IRR) calculated by generalized estimating equations. We constructed multivariable models adjusted for sex, age, comorbidities and medications. There were 20,634 new-onset community-acquired AKI cases. The frequency of new-onset community-acquired AKI gradually increased from June to July, with the highest adjusted IRR in July (1.19 [95% confidence intervals: 1.11-1.27]). Subgroup analyses showed that IRRs for community-acquired AKI were consistently higher in July than in February across all categories. Notably, a significant interaction was observed between seasonality and age (P = 0.029), with the highest IRR observed in the youngest age group (< 20 years). These results suggest the potential relevance of seasonal and environmental factors in AKI prevention in outpatient settings.