Urine in bladder denies clinically important dehydration?

Parental recall of urine output is poor.  Can we find a urine volume in the bladder that is associated with a well hydrated state? Or is there a rate of volume increase over a period of oral rehydration that can tell you the tank is full enough for discharge?  If we take a well hydrated cohort and give em oral fluid, will we find that their bladders fill at a significantly different rate than the slightly dry kids?

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