Background: Few epidemiologic studies on acute kidney injury (AKI) have focused on the older adult population. This study aimed to clarify the characteristics and risk factors for AKI in this population.

Methods: This retrospective observational study was performed with the clinical data of all outpatients and inpatients aged ≥ 65 years at the time of enrolment at Kochi Medical School Hospital between 1 January 1981 and 31 December 2021. The primary cohort was divided into those aged 65-74 and ≥ 75 years. The primary outcome was the occurrence of AKI.

Results: Of 83,822 patients, 38,333 were included in the 65-74-year-old group, whereas 45,489 were included in the ≥ 75-year-old group. Prevalences of the first AKI event in the 65-74-year-old and ≥ 75-year-old groups were 11.9% and 12.4%, respectively. Overall, lower estimated glomerular filtration rate, lower albumin level, lower or higher level of serum uric acid, and histories of diabetes mellitus, chronic heart failure, ischaemic heart disease, non-ischaemic heart disease, cerebrovascular disease, cancer, and liver disease were independent risk factors for an AKI event. The risk factors for AKI unique to each cohort were using non-steroidal anti-inflammatory drugs (NSAIDs) and loop diuretics (L-DI), and histories of hypertension (HT) and vascular diseases (VD) in men aged 65-74 years; using NSAIDs, angiotensin-converting enzyme inhibitors (ACEIs), L-DI and other diuretics (O-DI), and histories of HT and VD in men aged ≥ 75 years; using NSAIDs and O-DI and not using angiotensin-receptor blockers (ARBs), and a history of HT in women aged 65-74 years; and use of L-DI and a history of VD in women aged ≥ 75 years. Presence of proteinuria was a risk factor for developing AKI.

Conclusions: Many AKI risk factors reported thus far are associated with AKI development. However, there are differences in the effects of the renin-angiotensin system inhibitors, ACEIs, and ARBs (ARBs may be protective). Additionally, the U-shaped relationship between AKI onset and uric acid levels differs between sexes in the elderly population, similar to other age groups, but this sex difference disappears in the very elderly population. Pre-existing chronic kidney disease is a risk factor for the development of AKI.