Urinary phosphate/creatinine, calcium/creatinine, and magnesium/creatinine ratios in a healthy pediatric population☆,☆☆,★

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Abstract

Objective: To determine reference values for urinary phosphate/creatinine (Cr) concentration ratios and to complete reference values for urinary calcium/creatinine and magnesium/creatinine ratios in the second morning urine sample of healthy infants, children, and adolescents.

Design: Urinary P/Cr, Ca/Cr, and Mg/Cr ratios were determined from the second morning urine sample. Two urine samples were obtained 1 week apart from most subjects to assess reproducibility.

Setting: Kindergartens and schools of Lausanne, Switzerland.

Participants: A total of 410 healthy children aged 1 month to 17 years (197 girls and 213 boys) participated in the study.

Results: The 5th and 95th percentiles were estimated from 664 urine samples. There were no differences related to sex. A nonlinear regression in terms of age was used to smooth the estimated percentiles yielding reference curves from which critical values may be obtained for any given age. The 95th percentile for urinary Ca/Cr and Mg/Cr agreed with previously reported values in children older than 7 years. The upper limit of the three solute/creatinine ratios decreased significantly with age: for urinary P/Cr from 19.0 mol/mol at 1 month to 2.7 at 14 years; for urinary Ca/Cr from 2.2 to 0.7 mol/mol, and for urinary Mg/Cr from 2.2 to 0.6 mol/mol. Lower limits varied little. Interindividual and intraindividual variations decreased with age.

Conclusions: Urinary P/Cr, Ca/Cr, and Mg/Cr ratios vary strongly with age. We provide reference values, expressed both in SI and in mass units, for urinary P/Cr, Ca/Cr, and Mg/Cr in children aged one month to 17 years. (J Pediatr 1997;131:252-7)

Section snippets

Study Protocol

Children were told not to change their eating habits or physical activity, and a questionnaire relative to the family’s medical history was completed by the parents.

The second morning urine sample was collected between 8 and 11 am . For children who were not toilet-trained, a random urine sample was collected between 6 and 11 am . Adhesive urine collection bags were used to collect urine from infants. Whenever possible, two urine samples were collected under the same conditions at a 1-week

Results

On the basis of the following results, we decided not to segregate data and to determine reference values by age class according to the 664 available urine samples. First, there were no sex-related differences in the three solute/creatinine ratios. Second, two urine collections were obtained from 62% of subjects, and an order effect (systematic bias) between the first and the second urine samples could be excluded. Third, the number of urine samples collected from children who had not eaten

Discussion

The reference values proposed in this study were calculated for children older than 1 month. The mathematical model used to smooth the step function of the 95th percentiles makes possible an easy determination of usable upper limits for urinary P/Cr, Ca/Cr, and Mg/Cr at any given age. Few authors propose reference limits for mineral/creatinine ratios as percentiles, although this is an appropriate approach for asymmetric distributions. 14, 15 Because the interindividual and intraindividual

Acknowledgements

We thank Mrs. M. Hiller and all the personnel from the Central Laboratory of the Centre Hospitalier Universitaire Vaudois, Dr. Woringer and the nurses from the school medical services who helped organize the urine collections, as well as the educators from the kindergartens we visited. We also thank Drs. P.-A. Nicod, O. Carrel, P. Déglon, and L. Junier (pediatricians), for valuable help. We are particularly indebted to the children, adolescents, and their parents who participated in this study.

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    Supported by grant No. 32-36574.92 from the Swiss National Science Foundation.

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    Reprint requests: Vera Matos, MD, c/o Professeur Dr. Jean-Pierre Guignard, Unité de Néphrologie, Service de Pédiatrie, Centre Hospitalier Universitaire Vaudois, 1011 Lausanne, Switzerland.

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