Urinary sodium excretion in young infants: role of gestational and postnatal ages

Fractional urinary sodium excretion (FENa) and urinary sodium excretion (UNaE) were determined 88 times in 42 healthy, appropriate weight-for-gestational age infants. Gestational ages (GA) were 28-41 weeks; birthweights were 930-4135 g. Nineteen preterm infants were studied serially a total of 59 times between 1 and 55 days after birth. During the first 4 days after birth, multiple hierarchical regression analyses showed that FENa and UNaE were inversely related to GA (P less than 0.001). Postnatal age (PN) was not significantly related to FENa or UNaE. However the effect of GA on FENa and UNaE was not significantly greater than the effect due to postnatal age (PA) (P = 0.31 and P = 0.80, respectively). UNaE (1.6 +/- 0.2 mmol/kg per day) was significantly greater than sodium intake (1.1 +/- 0.2 mmol/kg per day) at 2.6 +/- 0.2 days (P less than 0.05). Longitudinal studies extending beyond 4 days indicated that GA and PA had interactive effects on FENa and UNaE; hence the contribution for a given GA was dependent on PA (and vice versa). Sodium intake remained stable (average 1.8 mmol/kg per day) beyond 7 days after birth and was consistently greater than UNAE (P less than 0.01). It is suggested that in healthy preterm infants beyond 2 weeks PA, a sodium intake of 2-3 mol/kg per day may be adequate to meet the renal sodium losses.