Bladder capacity as measured at voiding cystourethrography in children: relationship to toilet training and frequency of micturition.

To assess the reliability of a frequently used formula for calculating bladder capacity in children, bladder capacity was measured prospectively at voiding cystourethrography in 274 consecutive healthy infants and children. Bladder volume index (BVI) was then calculated by dividing the measured capacity by the predicted capacity. Bladder capacity increased dramatically after 18 months and reached a plateau between 3 and 4 months of age, after which there was no further significant increase in capacity until approximately 9 years. BVI abruptly increased from 1.03 in children aged less than 18 months to 1.50-1.60 in children aged between 18 months and 4 years (P < or = .0001). Because bladder capacity increases sharply during infancy and early childhood and levels off once toilet training is complete, linear models used to predict bladder capacity on the basis of age alone significantly underestimate bladder capacity in infants and younger children.