Acta PRdiatr Scand 79: 1 156-1 162, 1990
Urinary Creatinine Excretion and Estimation of Muscle Mass in Infants of 25-34 Weeks Gestation N. MOD1 and J. L. HUTTON From the Department of Paediatrics and the Department of Community Health, University of Liverpool, Liverpool. U.K.
ABSTRACT. Modi, N. and Hutton, J. L. (Department of Paediatrics and Department of Community Health, University of Liverpool, Liverpool, United Kingdom). Urinary creatinine excretion and estimation of muscle mass in infants of 25-34 weeks gestation. Acta Paediatr Scand 79: 1156, 1990. The urinary excretion rate of creatinine was measured over 89 days in 31 infants of gestational ages 25-34 weeks in the first week of life. Creatinine excretion was shown to have a significant positive correlation with weight and postconceptional age. Creatinine excretion factored by body weight was also positively correlated with weight and postconceptional age. Pooled analysis of published data together with our own, confirms that this correlation is important. Muscle mass was estimated from creatinine excretion rate. A regression equation was derived predicting muscle mass from birthweight and gestational age. Muscle mass increases from 12% of birthweight at 25 weeks gestation to 19 % at 34 weeks and 24 % at 40 weeks. This is in agreement with classic dissection studies which showed muscle mass to be 25% of body weight at term. Key words: creatinine excretion, muscle mass, neonate, preterm infant.
Creatinine is derived from phosphocreatine by a non enzymatic process occurring in muscle. In 1947 Borsook & Dubnoff (1) reported that 98% of total body creatinine is located in muscle and that 2 % is converted daily to creatinine and excreted in the urine. At steady state therefore, creatinine excretion is an indirect measure of muscle mass. Creatinine excretion factored by body weight is similarly a reflection of the relative amount of muscle in the body (2). A knowledge of urinary creatinine excretion has several applications; it has been used as an index of muscle mass and nutritional status (3), to estimate urine flow rate and to relate the excretion of other metabolites (4). There is limited information relating to creatinine excretion in extremely immature infants. This paper presents data relating to a typical group of preterm infants.
METHODS Continuous urine collections were obtained in male infants during the first week of life. These infants were receiving varying degrees of intensive care support but had no evidence of any abnormality in renal function; all had urine flow rates in excess of 0.5 ml kg-' h-' and in none was there a rise in plasma creatinine during the period of the study. Gestational age was assessed from the menstrual history, antenatal ultrasound measurements and the clinical scoring system of Dubowitz et al. (5). Urine was collected using a closed circuit system consisting of a small plastic pouch applied to the infant's perineum with medical adhesive following the application of a skin protector (Dow Corning); a plastic tube inserted into the pouch was connected to a collection bottle via a low grade suction pump. Collections were continued for as long as possible until a leakage occurred; the precise duration of each collection was noted in minutes. Each infant was weighed daily by one of the investigators (NM); care was taken to obtain weights net of endotracheal tubes, intravenous cannulae and other attachments. Urinary
Acta Paediatr Scand 79
Urinary creatinine excretion in the newborn 1 157
creatine was assayed by the kinetic rate Jaffe method. The between batch coefficient of variation for urinary creatinine assay in our laboratory is 2.8%. Daily urinary creatinine excretion was calculated from a knowledge of daily urine output and urinary creatinine concentration. Muscle mass was estimated using the factor of 20 suggested by Graystone (2); i.e. the excretion of 1 g of creatinine per 24 hours derived from 20 kg muscle (1 mg = 8.8 pmol creatinine). The relationships between daily creatinine excretion and postconceptional age, daily creatinine excretion and weight, daily creatinine excretion factored by bodyweight and postconceptional age and daily creatinine excretion factored by bodyweight and weight, were studied. As repeated measurements were made (6) analyses of variance and covariance were used to examine within and between infant differences. Values for daily creatinine excretion and daily creatinine excretion factored by body weight are analysed following logarithmic transformation to stabilise the variances. The influence of postnatal age was assessed on daily creatinine excretion and found not to vary within the study period of seven days allowing for between infant differences and actual weight; data from each infant was therefore aggregated to provide a single mean daily creatinine excretion rate for each infant in order to estimate muscle mass as described above. These results are analysed using simple linear regression. Data analysis was performed using SPSSX. The study had the approval of the hospital ethical committee.
RESULTS Continuous urine collections were obtained over 89 days in 3 1 male babies during the first week of life. Birthweight ranged from 680 to 1830 g and gestational age from 25 to 34 weeks. The median duration of each total urine collection was 3 465 min (mean 3 744, range 630-9 930) and the median duration of each daily urine collection 1425 min (mean 1276, range 105-1 800). The median daily creatinine excretion was 71.4 pmol kg-I day-' (mean 71.4, 95% confidence interval 65.2-77.6). The median muscle mass as a percentage of birthweight was 14.3 O/o (mean 15, 95 O/o confidence interval 13.4-1 6.0). Log creatinine excretion (pmol day-') was found to have a significant positive association with actual weight (F= 5.8, p
Urinary Creatinine Excretion and Estimation of Muscle Mass in Infants of 25-34 Weeks Gestation N. MOD1 and J. L. HUTTON From the Department of Paediatrics and the Department of Community Health, University of Liverpool, Liverpool. U.K.
ABSTRACT. Modi, N. and Hutton, J. L. (Department of Paediatrics and Department of Community Health, University of Liverpool, Liverpool, United Kingdom). Urinary creatinine excretion and estimation of muscle mass in infants of 25-34 weeks gestation. Acta Paediatr Scand 79: 1156, 1990. The urinary excretion rate of creatinine was measured over 89 days in 31 infants of gestational ages 25-34 weeks in the first week of life. Creatinine excretion was shown to have a significant positive correlation with weight and postconceptional age. Creatinine excretion factored by body weight was also positively correlated with weight and postconceptional age. Pooled analysis of published data together with our own, confirms that this correlation is important. Muscle mass was estimated from creatinine excretion rate. A regression equation was derived predicting muscle mass from birthweight and gestational age. Muscle mass increases from 12% of birthweight at 25 weeks gestation to 19 % at 34 weeks and 24 % at 40 weeks. This is in agreement with classic dissection studies which showed muscle mass to be 25% of body weight at term. Key words: creatinine excretion, muscle mass, neonate, preterm infant.
Creatinine is derived from phosphocreatine by a non enzymatic process occurring in muscle. In 1947 Borsook & Dubnoff (1) reported that 98% of total body creatinine is located in muscle and that 2 % is converted daily to creatinine and excreted in the urine. At steady state therefore, creatinine excretion is an indirect measure of muscle mass. Creatinine excretion factored by body weight is similarly a reflection of the relative amount of muscle in the body (2). A knowledge of urinary creatinine excretion has several applications; it has been used as an index of muscle mass and nutritional status (3), to estimate urine flow rate and to relate the excretion of other metabolites (4). There is limited information relating to creatinine excretion in extremely immature infants. This paper presents data relating to a typical group of preterm infants.
METHODS Continuous urine collections were obtained in male infants during the first week of life. These infants were receiving varying degrees of intensive care support but had no evidence of any abnormality in renal function; all had urine flow rates in excess of 0.5 ml kg-' h-' and in none was there a rise in plasma creatinine during the period of the study. Gestational age was assessed from the menstrual history, antenatal ultrasound measurements and the clinical scoring system of Dubowitz et al. (5). Urine was collected using a closed circuit system consisting of a small plastic pouch applied to the infant's perineum with medical adhesive following the application of a skin protector (Dow Corning); a plastic tube inserted into the pouch was connected to a collection bottle via a low grade suction pump. Collections were continued for as long as possible until a leakage occurred; the precise duration of each collection was noted in minutes. Each infant was weighed daily by one of the investigators (NM); care was taken to obtain weights net of endotracheal tubes, intravenous cannulae and other attachments. Urinary
Acta Paediatr Scand 79
Urinary creatinine excretion in the newborn 1 157
creatine was assayed by the kinetic rate Jaffe method. The between batch coefficient of variation for urinary creatinine assay in our laboratory is 2.8%. Daily urinary creatinine excretion was calculated from a knowledge of daily urine output and urinary creatinine concentration. Muscle mass was estimated using the factor of 20 suggested by Graystone (2); i.e. the excretion of 1 g of creatinine per 24 hours derived from 20 kg muscle (1 mg = 8.8 pmol creatinine). The relationships between daily creatinine excretion and postconceptional age, daily creatinine excretion and weight, daily creatinine excretion factored by bodyweight and postconceptional age and daily creatinine excretion factored by bodyweight and weight, were studied. As repeated measurements were made (6) analyses of variance and covariance were used to examine within and between infant differences. Values for daily creatinine excretion and daily creatinine excretion factored by body weight are analysed following logarithmic transformation to stabilise the variances. The influence of postnatal age was assessed on daily creatinine excretion and found not to vary within the study period of seven days allowing for between infant differences and actual weight; data from each infant was therefore aggregated to provide a single mean daily creatinine excretion rate for each infant in order to estimate muscle mass as described above. These results are analysed using simple linear regression. Data analysis was performed using SPSSX. The study had the approval of the hospital ethical committee.
RESULTS Continuous urine collections were obtained over 89 days in 3 1 male babies during the first week of life. Birthweight ranged from 680 to 1830 g and gestational age from 25 to 34 weeks. The median duration of each total urine collection was 3 465 min (mean 3 744, range 630-9 930) and the median duration of each daily urine collection 1425 min (mean 1276, range 105-1 800). The median daily creatinine excretion was 71.4 pmol kg-I day-' (mean 71.4, 95% confidence interval 65.2-77.6). The median muscle mass as a percentage of birthweight was 14.3 O/o (mean 15, 95 O/o confidence interval 13.4-1 6.0). Log creatinine excretion (pmol day-') was found to have a significant positive association with actual weight (F= 5.8, p
