Plasma and urine osmolality in full-term and pre-term infants

The purpose of the present study was to determine the normal relationship of urine and plasma osmolality in pre-term and full-term infants. Twenty-nine full-term and 35 pre-term babies were studied during the first week of life. Random simultaneous urine and plasma samples were measured for osmolality. Statistical analysis defined the normal relationship of plasma to urine osmolality. This suggests an osmotic threshold of 282 mOsm/kg and 291 mOsm/kg for full-term and pre-term babies, respectively. These values are different from the 285-290 mOsm/kg of adolescents and adults. The nomograms provided may serve as a useful aid in the bedside diagnosis of dysfunctional secretion of vasopressin.