Incomplete bladder emptying cannot be diagnosed on a single post-void residual urine on ultrasound, due to significant intra-individual variability. Two post-void residual urine tests are recommended; larger volumes are seen if the bladder has been over distended (eg initial volume greater than 115% of expected), and in younger children. Greater than 20 ml is more specific than 10% bladder capacity. [J urology 2009 (182):1933]
Bladder capacity is = (Age +1) x 30 (ml) max 390ml.