Pediatric Nephrology
Pediatr Nephrol (1990) 4: 105- 110 9 IPNA 1990
Original article Transtubular potassium concentration gradient: a useful test to estimate renal aldosterone bio-activity in infants and children Juan Rodriguez-Soriano, Mercedes Ubetagoyena, and Aifredo Vallo Division of Paediatric Nephrology, Department of Paediatrics, Hospital Infantil de Cruces and Basque University School of Medicine, Bilbao, Spain Received June 1/Receivedin revised form September 27/Accepted October 19, 1989
Abstract. The present investigation was designed to validate the usefulness of transtubular potassium (K) concentration gradient (TTKG) as an indicator of aldosterone bio-activity in infants and children. T T K G was calculated by the formula: [K]urine: (urine/plasma)osmolality [K]venous blood, We compared this index with fractional K excretion (FEN) and urine K concentration to urine sodium (Na) concentration ratio (UK/UNa)ill 473 normal children aged 1 m o n t h 15 years. Values of T T K G followed a non-ganssian distribution (median, 6.3; 3rd centile, 4.1; 97th centile, 13.4). T T K G in infants (n=108; median, 7.8) was significantly higher than in children (n=365; median, 6.0). T T K G correlated directly with FEK and UK/UNa.Indices of K excretion were also assessed in 13 patients with hypo- and pseudohypoaldosteronism. T T K G values varied between 1.6 and 4.1 and were all below the 3rd percentile established for the age of the subject. We conclude that calculation of T T K G is an easy and sensitive method for the evaluation of mineralocorticoid action in distal and collecting tubules.
tion and excretion or on the calculation of fractional K excretion (FEK) and ratio of urine K concentration to urine sodium (Na) concentration (UK/UNa) [1, 2]. Recently, West et al. [3, 4] have proposed a formula for estimating the transtubular K concentration gradient (TTKG) in the cortical distal nephron, and thereby aldosterone bio-activity. Renal K excretion is the product of luminal K concentration and flow rate in distal and cortical collecting tubules; but only the former is influenced by aldosterone [5]. Since luminal K concentration is further influenced by removal of water in the medullary segments, an estimation of cortical lumen K concentration can be made by dividing the urine K concentration by the urine-to-plasma (U/P) osmolality ratio [3, 4]. Calculation of T T K G has been found clinically useful for evaluating states of low aldosterone bio-activity in adults [6], but there are no studies applying this test to children. The present investigation was designed both to determine the range of normal T T K G values and to assess its validity as an indicator of renal tubular action of aldosterone in children with hypo- or pseudohypoaldosteronism.
Key words: Potassium - Renal potassium excretion Aldosterone - Hypoaldosteronism - Pseudohypoaldosteronism
Subjects and methods Introduction The assessment of renal handling of potassium (K) is necessary to evaluate the tubular effect of aldosterone in patients with sustained hyperkalaemia. The traditional approach focuses either on the study of urinary K concentra-
Offprint requests to: J. Rodrfguez-Sofiano
FEK, UKAJNaand TTKG were calculated using data from renal function studies, previously carried out in our laboratory for various diagnostic reasons. Only patients showing no signs of glomerularor tubular impairment were selected. These patients had normal plasma K concentration and were considered to have a normal electrolyte metabolism. Most data represent follow-up studies of patients referred because of haematuria, proteinuria or urinary tract infection. All subjects were on a standard, uncontrolled diet and received no medication. The population studied (240 males, 233 females) included 108 infants from 1 to 12 months and 365 children from 1 to 15 years (mean age + SD, 5.4 + 4.4 years). Data collected from 13 infants and children diagnosed as having hypo- or pseudohypoaldosteronismwere also reviewed. Amongst them,
106 Table 1. Biochemical data and indices of renal potassium (K) excretion in infants and children
PNa(mEq/1)a PK(mEq/1) ~
Infants (n = 1 0 8 )
Children (n : 365)
P
139.1 + 2.8
140.0 _+ 2.3
Pediatr Nephrol (1990) 4: 105- 110 9 IPNA 1990
Original article Transtubular potassium concentration gradient: a useful test to estimate renal aldosterone bio-activity in infants and children Juan Rodriguez-Soriano, Mercedes Ubetagoyena, and Aifredo Vallo Division of Paediatric Nephrology, Department of Paediatrics, Hospital Infantil de Cruces and Basque University School of Medicine, Bilbao, Spain Received June 1/Receivedin revised form September 27/Accepted October 19, 1989
Abstract. The present investigation was designed to validate the usefulness of transtubular potassium (K) concentration gradient (TTKG) as an indicator of aldosterone bio-activity in infants and children. T T K G was calculated by the formula: [K]urine: (urine/plasma)osmolality [K]venous blood, We compared this index with fractional K excretion (FEN) and urine K concentration to urine sodium (Na) concentration ratio (UK/UNa)ill 473 normal children aged 1 m o n t h 15 years. Values of T T K G followed a non-ganssian distribution (median, 6.3; 3rd centile, 4.1; 97th centile, 13.4). T T K G in infants (n=108; median, 7.8) was significantly higher than in children (n=365; median, 6.0). T T K G correlated directly with FEK and UK/UNa.Indices of K excretion were also assessed in 13 patients with hypo- and pseudohypoaldosteronism. T T K G values varied between 1.6 and 4.1 and were all below the 3rd percentile established for the age of the subject. We conclude that calculation of T T K G is an easy and sensitive method for the evaluation of mineralocorticoid action in distal and collecting tubules.
tion and excretion or on the calculation of fractional K excretion (FEK) and ratio of urine K concentration to urine sodium (Na) concentration (UK/UNa) [1, 2]. Recently, West et al. [3, 4] have proposed a formula for estimating the transtubular K concentration gradient (TTKG) in the cortical distal nephron, and thereby aldosterone bio-activity. Renal K excretion is the product of luminal K concentration and flow rate in distal and cortical collecting tubules; but only the former is influenced by aldosterone [5]. Since luminal K concentration is further influenced by removal of water in the medullary segments, an estimation of cortical lumen K concentration can be made by dividing the urine K concentration by the urine-to-plasma (U/P) osmolality ratio [3, 4]. Calculation of T T K G has been found clinically useful for evaluating states of low aldosterone bio-activity in adults [6], but there are no studies applying this test to children. The present investigation was designed both to determine the range of normal T T K G values and to assess its validity as an indicator of renal tubular action of aldosterone in children with hypo- or pseudohypoaldosteronism.
Key words: Potassium - Renal potassium excretion Aldosterone - Hypoaldosteronism - Pseudohypoaldosteronism
Subjects and methods Introduction The assessment of renal handling of potassium (K) is necessary to evaluate the tubular effect of aldosterone in patients with sustained hyperkalaemia. The traditional approach focuses either on the study of urinary K concentra-
Offprint requests to: J. Rodrfguez-Sofiano
FEK, UKAJNaand TTKG were calculated using data from renal function studies, previously carried out in our laboratory for various diagnostic reasons. Only patients showing no signs of glomerularor tubular impairment were selected. These patients had normal plasma K concentration and were considered to have a normal electrolyte metabolism. Most data represent follow-up studies of patients referred because of haematuria, proteinuria or urinary tract infection. All subjects were on a standard, uncontrolled diet and received no medication. The population studied (240 males, 233 females) included 108 infants from 1 to 12 months and 365 children from 1 to 15 years (mean age + SD, 5.4 + 4.4 years). Data collected from 13 infants and children diagnosed as having hypo- or pseudohypoaldosteronismwere also reviewed. Amongst them,
106 Table 1. Biochemical data and indices of renal potassium (K) excretion in infants and children
PNa(mEq/1)a PK(mEq/1) ~
Infants (n = 1 0 8 )
Children (n : 365)
P
139.1 + 2.8
140.0 _+ 2.3
