Residual hydronephrosis after paediatric pyeloplasty: longitudinal APD reference curves.

PURPOSE: Residual hydronephrosis is common after pyeloplasty and can complicate follow-up. We developed renal pelvic anteroposterior diameter (APD) reference curves to help interpret postoperative ultrasound findings and identify renal units that merit closer review. METHODS: We retrospectively reviewed children undergoing primary pyeloplasty for isolated pelvi-ureteric junction obstruction between February 2016 and September 2020. Curves derived from renal units without recurrence described postoperative APD according to preoperative APD and time since surgery. A mixed-effects model accounted for repeated measurements from each renal unit. We compared reoperation rates in units with any measurement above the 90th-centile upper reference curve and those remaining below it. RESULTS: The cohort included 566 renal units in 550 children. Reference curves used 1,229 measurements from 551 units without recurrence, with median follow-up of 17.0 months. APD decreased most during the early postoperative months and remained higher in renal units with greater preoperative dilatation. At 6 months, the upper reference APD was 23.0 mm for a preoperative APD of 15 mm and 38.4 mm for 70 mm. Among 564 units with documented outcomes, reoperation occurred in 9 of 77 units above the curve (11.7%) and 4 of 487 below it (0.8%; risk ratio 14.23, 95% confidence interval 4.49-45.08). A web calculator compares postoperative APD with the corresponding reference values from 3 to 24 months. CONCLUSION: The curves provide a reference for interpreting residual hydronephrosis using routine follow-up ultrasound measurements. APD above the upper curve was associated with more frequent reoperation and can help identify children for earlier reassessment.