Plasma urea and creatinine concentrations and urea and creatinine clearances and excretion were measured in a sample of 40 infants of 25.5-33 weeks' gestation, birth weight 720-2000 g, between the ages of 0.5 and 33 days. Creatinine excretion rate was between 60 and 120 mumol/kg/day in the first five postnatal weeks (mean 90.5) and was independent of sex or growth retardation. This can be used in clinical practice to estimate instantaneous urine flow rate V, if the creatinine concentration is measured in a randomly voided urine sample, from the formula V = 90.5/urine creatinine, with 95% confidence limits +/- 39%. There is a wide range of plasma creatinine at all gestations and ages decreasing from range 75-130 mumol/l in the first two days to 35-80 mumol/l at 3 weeks of age. Plasma urea is a poor indicator of glomerular filtration rate (GFR) in sick preterm infants. GFR (ml/min/kg) can be estimated from plasma creatinine from the formula GFR = 69.2/plasma creatinine but this estimate is imprecise with 95% confidence limits +/- 46%. Urea:creatinine clearance ratio was usually less than 1.0 (range 0.18 to 1.5) and was lower when the urine flow rate was low. Urea excretion was up to 17 mmol/kg/day in the first two weeks, higher in the more immature infants. These high levels were paralleled by a high plasma urea concentration, up to 18 mmol/l. A high plasma urea is not necessarily associated with renal failure or dehydration.
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