Renal function in sick very low birthweight infants: 1. Glomerular filtration rate.

A total of 135 measurements of polyfructoside clearance as a measure of glomerular filtration rate (GFR) were made in 39 infants of 25.5-33 weeks' gestation, birth weight 720-2000 g, between the ages of 0.5 and 33 days. GRF was related to postconceptional age and increased exponentially from geometric mean 0.59 ml/min at 26 weeks' postconceptional age to 1.40 ml/min at 33 weeks. GFR in the first week and GFR at later ages were the same for a given postconceptional age. GFR was the same in sick infants with severe ventilatory failure as in less ill infants. There was no evidence that GFR was influenced by nitrogen input. GFR increases postnatally in a preprogrammed way irrespective of other postnatal events. When factored by body weight GFR in the first week increased only little from arithmetic mean 0.70 ml/min/kg at 26 weeks to 0.84 ml/min/kg at 33 weeks, but older infants often had a falsely high GFR per kg when they lost weight in the first week or two after birth or failed to gain weight later.