Acute kidney injury (AKI), previously called acute renal failure (ARF), is a sudden decrease in kidney function that develops within 7 days, as shown by an increase in serum creatinine or a decrease in urine output, or both. The diagnosis of AKI is made based on a person's signs and symptoms, along with lab tests for serum creatinine and measurement of urine output. Other tests include urine microscopy and urine electrolytes. Renal ultrasound can be obtained when a postrenal cause is suspected. A kidney biopsy may be obtained when intrinsic renal AKI is suspected and the cause is unclear.