Urea and creatinine excretion

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 gmol/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 ±390/o. There is a wide range of plasma creatinine at all gestations and ages decreasing from range 75-130 gmol/1 in the first two days to 35-80 gmol/l at 3 weeks of age. Plasma urea is a poor indicator of glomerular filtration rate (GFR) in sick preterm infants. GFR (mi/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 10 (range 0-18 to 1.5) and was lower when the urine flow rate was low. Urea excretion was up to 17 mmol/kglday 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. (Arch Dis Child 1992;67:1 146-53) Department of Child Health, Bristol University and Neonatal Intensive Care Unit, Southmead Hospital, Bristol Correspondence to: Dr B H Wilkins, Department of Paediatrics, Westmead Hospital, Westmead, NSW 2145, Australia. Accepted 24 April 1992 Reprints not available. The clinical management of sick preterm infants requires meticulous attention to many aspects of physiology and to the daily prescribing' of water, salts, and nutrition. Renal physiology often receives scant attention because plasma creatinine is the only accessible index of renal function, and even this may be unreliable because of interfering substances in routine methods. Although urine is easily obtainable it is rare for neonatal units to measure urinary indices of renal function routinely. In prescribing intravenous water and electrolyte treatment it may be useful to have a day to day, or even more frequent, estimate of renal excretion of water, sodium, potassium, etc. Such an estimate may be obtained without the tedium and inaccuracy of timed urine collections by relating urine concentration to creatinine, provided creatinine excretion rate is not scattered too widely. The few reports of creatinine excretion so far are not encouraging in this respectl-6 but most have relied on timed urines for their measurement. Plasma urea is a poor guide to glomerular function in all subjects but has received little attention in the sick preterm newborn. The purpose of the present investigation was: (1) to establish a normal range for plasma creatinine in sick infants less than 33 weeks' gestation at various ages using a creatinine method which measures true creatinine without interference; (2) to establish a normal range for creatinine excretion and thereby derive formulas to estimate glomerular filtration rate (GFR) and urine flow rate from plasma and urine creatinine; and (3) to investigate the usefulness of plasma urea as an indicator of renal function by examining the relationship between urea and creatinine clearance and between plasma urea and urea excretion. Patients and methods This study was part of a wider study of glomerular and tubular function in very low birthweight infants. Altogether 40 infants were chosen who could be studied in the first postnatal week, and later if possible. Gestation at birth was 25-5-33 weeks, birth weight 720-2000 g. Further clinical, experimental, and laboratory details are given in part 1.7 Plasma creatinine and urea concentrations were measured sequentially in 40 infants up to age 3-33 days. Urine flow rate was measured on 124 occasions in 39 infants by a continuous infusion Polyfructosan-S (PF-S, Laevosan-Gesellschaft) clearance method,8 9 between the ages of 0 5 and 33 days. Urine flow rate (urine volume or renal water excretion rate, ml/day) was calculated by: Urine flow rate=PF-S infusion rate/urine PF-S concentration Creatinine excretion rate (,umol/day) was calculated by: Creatinine excretion=urine flow ratexurine creatinine concentration Urea and sodium excretion rates (mmol/day) were similarly calculated. Plasma and urine creatinine were measured by a modification8 of a resin adsorption method10 11 which avoids the positive interference of non-creatinine chromogen and the negative interference of 1146 group.bmj.com on June 6, 2017 Published by http://adc.bmj.com/ Downloaded from