Acute kidney injury (AKI), also known as acute renal failure (ARF), is a sudden episode of kidney failure or kidney damage that happens within a few hours or a few days. AKI causes a build-up of waste products in your blood and makes it hard for your kidneys to keep the right balance of fluid in your body. AKI can also affect other organs such as the brain, heart, and lungs. Acute kidney injury is common in patients who are in the hospital, in intensive care units, and especially in older adults.