During the first appointment to my office I ask the child to drink enough water that he or she will be able to pee at least two or more times. I do an ultrasound of the bladder and rectum before and after each void. The child is instructed to pee into a Uroflow device that measures peak flow rate, voiding time, and the volume of pee voided. The post-void ultrasound allows me to assess bladder emptying. Together with a good history, the data allows a very good non-invasive assessment of bladder function.
The kidneys are responsible for maintaining the water balance in the body. When the child is dehydrated the kidneys make very little urine. Once a child is optimally hydrated and the body has all the water necessary for health, the kidneys make urine as fast as the child drinks water.
Almost every child with day or night time wetting has a bladder capacity that is smaller than average. Most have a bladder capacity that is less than half of average and some have a bladder capacity that is only a quarter or a third of average. Once optimally hydrated, a child with a very small capacity might pee as often as four or more times over the course of an hour.
Most children with day and night time wetting have borderline or poor hydration. These children do not drink very much for a variety of reasons that make sense to either the child or the adult caregivers. Parents often limit drinking in the evening. Children self limit drinking at school to avoid either voiding too often or to minimise the risk of daytime wetting.
The urine in a child who is dehydrated is dark yellow (concentrated). The kidneys have a limited ability to concentrate the urine. In a dehydrated state, without enough water in the body, the kidneys are not able to promptly and optimally excrete all of the waste products that have built up after the food is metabolized by the body.
The urine in a child who is well hydrated is clear (dilute). Once optimally hydrated, the water that a child drinks is quickly made into urine. The kidneys can dilute the urine to make almost pure water.
Once every month or so, a well-hydrated child in the office comments that there is something "wrong" with the colour of their urine. The child tells their parents that the pee is "white." This implies that until this moment, he or she has never seen dilute (clear) urine. This implies that their hydration has always been poor or borderline.
I routinely point out and discuss the dark urine voided by children who attend my office. I recommend that children drink enough water early in the day so that their urine is clear (dilute) before lunch.