The glomerular filtration rate (GFR) of neonates is 1/3 - 1/4 of the mature individual.

Tubular immaturity exists. The neonate can only concentrate urine to an osmolality 1 & 1/2 times plasma osmolality whereas adult dogs and cats can concentrate urine to several times the osmolality of plasma.

Fluid intake and excretion of fluids are 5 - 7 times as great in relationship to weight compared to adults making the neonate more susceptible to fluid depletion.

The metabolic rate of the neonate is 2 - 2 1/2 times greater than adults in relation to body mass

Water consumption of the neonate is ~60 - 90 ml/lb/day compared to the average water intake of the adult which is ~20-30 ml/lb/day. This needs to be taken into consideration when formulating a fluid therapy plan for a neonatal animal.

Until 8 weeks of age the urine specific gravity is in the range of 1.006 - 1.017. Adult animals often have a urine specific gravity in excess of 1.035.

A small amount of glucose may be present in the urine of puppies and kittens (glucosuria) up to 8 weeks of age despite the presence of a normal blood glucose.

Because of renal immaturity and high water turnover, neonates are very susceptible to fluid imbalance and intoxication with renally excreted substances. These factors must be taken into consideration when formulating a fluid therapy plan for a neonatal animal. If you have a choice of drugs to use when treating a neonatal animal, avoid potentially nephrotoxic drugs. If nephrotoxic drugs must be used, maintain hydration and keep the duration of treatment as short as possible.