MEASURED VS CALCULATED PLASMA OSMOLALITY IN INFANTS WITH VERY LOW BIRTH WEIGHTS

Objective. - To determine the relationship between measured serum osmolality (MsOsm) and calculated osmolality and to examine factors that may affect the osmolal gap. Research Design. - Longitudinal cohort study. Setting. - Regional tertiary neonatal intensive care nursery in a university-affiliated hospital. Patients. - Sixty low-birth-weight infants (birth weight, 540 to 1500 g), studied daily during the first week of life. Selection Procedure. - Consecutive sample. Interventions. - None. Measurements and Results. - The MsOsm was significantly higher than the calculated osmolality for the first 6 days of life. The MsOsm was significantly higher during the first 6 days of life in infants with birth weights less than 1000 g than in those with birth weights greater than 1000 g, but the calculated osmolality was similar in both groups. Intraventricular hemorrhage, preservative additives in drugs, and packed red blood cell transfusions did not contribute significantly to osmolal gap or MsOsm. In 19 patients, peak MsOsm was greater than or equal to 320 mmol/kg (mean, 336 +/- 13 mmol/kg; calculated osmolality, 298 +/- 20 mmol/kg; osmolal gap, 38 +/- 19 mmol/kg). Six of these 19 patients died (all with birth weights < 1000 g). Conclusions. - A significant proportion of patients with very low birth weights (mostly < 1000 g) have large osmolal gaps and/or an MsOsm greater than 300 mmol/kg during the first week of life. The relationship between increased MsOsm in infants with very low birth weights and effective osmolality requires further study. Therapeutic intervention based solely on elevated MsOsm is ill advised.