Define oliguria and anuria by urine output and describe their clinical significance in acute kidney injury.

• A. Urine output < 400 mL/day
• B. Urine output < 800 mL/day
• C. Urine output < 1000 mL/day
• D. Urine output < 60 mL/day

Answer: (A)

Oliguria is a noticeable drop in urine production. In adults, it’s typically defined as urine output less than about 400 mL per day. Anuria is a more severe form, with almost no urine production, generally defined as less than around 100 mL per day.

In acute kidney injury, these thresholds matter because reduced urine output signals impaired kidney function and helps gauge severity. Oliguria often reflects problems in renal perfusion, tubular injury, or obstruction, and it means the kidneys aren’t processing fluids and waste as they should. Clinically, it raises concern for azotemia and potential electrolyte and fluid balance issues, such as hyperkalemia, metabolic acidosis, and volume overload. This recognition prompts evaluating fluid status and perfusion, reviewing nephrotoxic exposures, and checking for obstruction with imaging. If urine output remains low despite optimization, it suggests more severe AKI and may lead to consideration of renal replacement therapy. Anuria indicates particularly severe impairment and is associated with a worse prognosis, underscoring the need for urgent evaluation and management.