Variability, related factors and normal reference value of post-void residual urine in healthy kindergarteners

We report intra-individual variability, related factors and the normal reference value of post-void residual urine in children.

Healthy kindergarteners were enrolled for the evaluation of post-void residual urine, which was assessed by ultrasound within 5 minutes after voiding when voided volume was greater than 50 ml. Bladder over distention was arbitrarily defined as a bladder capacity (voided volume plus post-void residual urine) of 115% or greater of expected bladder capacity. The Pearson correlation method was used to evaluate the correlation between post-void residual urine and related factors.

More than 1 post-void residual urine value was recorded in 219 children with a mean +/- SD age of 4.9 +/- 0.9 years. Mean post-void residual urine was 12.2 +/- 20.3 ml (median 5.5). The correlation for consecutive post-void residual urine was low in all children and negligible in the 129 without bladder over distention (r = 0.34 and 0.13, respectively). Repeat post-void residual urine greater than 20 ml and greater than 10% bladder capacity was observed in 2.3% and 7.8% of children, respectively, without bladder over distention. Post-void residual urine increased as bladder capacity increased (r = 0.38, p <0.01). Excluding those with bladder over distention, post-void residual urine decreased as the age of the child increased. Children who drank more fluids before voiding had a higher rate of bladder over distention for each micturition than those who drank regularly (13.8% vs 4.9%, p = 0.04).

Because of the significant intra-individual variability of post-void residual urine, a single post-void residual urine test is not reliable for assessing pediatric voiding function. Two post-void residual urine tests are recommended. Post-void residual urine is affected by bladder over distention, age of the child and possibly extra hydration before assessment. Abnormal post-void residual urine could be defined as post-void residual urine greater than 20 ml, rather than as greater than 10% bladder capacity, on repeat micturitions without bladder over distention.