Oliguria, anuria and urinary obstruction: Viva

Branch 1: Definition and classification

The candidate should recognise that this infant has true anuria with a lower tract obstruction, the classic presentation of posterior urethral valves in a male infant. A strong candidate states the urine output thresholds precisely: oliguria is a urine output below 0.5 mL per kg per hour for six hours or more, and anuria is below 0.3 mL per kg per hour for 24 hours or more, or complete absence for 12 hours or more, by the KDIGO criteria. The candidate should note the stricter bedside convention of anuria as below 0.1 mL per kg per hour or below 1 mL per kg per day, and that the absence of any urine on catheterisation distinguishes true anuria from retention or a missed collection. ^[1]

If the examiner presses on the classification, the candidate should describe the three anatomical levels: pre-renal, where reduced perfusion lowers the filtration pressure in a structurally intact kidney; renal or intrinsic, where the nephron is structurally damaged; and post-renal or obstructive, where a block in the outflow tract raises the hydrostatic pressure upstream. The candidate should identify this infant as a post-renal cause, and should name posterior urethral valves as the most common cause of lower tract obstruction in male infants, presenting with a palpable distended bladder, a poor stream, and antenatal bilateral hydronephrosis. ^[6]