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Specificity of pH and osmolality of early morning urine sample in assessing distal renal tubular function in children: results in healthy children

BMJ 1996; 312 doi: http://dx.doi.org/10.1136/bmj.312.7042.1337 (Published 25 May 1996) Cite this as: BMJ 1996;312:1337
  1. Roderick Skinner, MRC training fellowa,
  2. Michael Cole, research associate (medical statistics)a,
  3. Andrew D J Pearson, senior lecturer in paediatric oncologya,
  4. Malcolm G Coulthard, consultant paediatric nephrologista,
  5. Alan W Craft, professor of paediatric oncologya
  1. a Sir James Spence Institute of Child Health, University of Newcastle upon Tyne, Royal Victoria Infirmary, Newcastle upon Tyne NE1 4LP
  1. Correspondence to: Dr Skinner.
  • Accepted 9 January 1996

A simple method of evaluating distal tubular function in children would be useful because water deprivation tests are potentially dangerous and acid loading tests are unpleasant. Osmolality of an early morning urine sample is often taken as a measure of urinary concentration in children with a history of polydipsia and polyuria, and the pH of an early morning sample is used to evaluate urinary acidification in several renal and metabolic disorders. A pH of 5.4 or less is usually taken as adequate acidification1 and an osmolality of 600 mmol/kg or more as excluding clinically significant impairment of urinary concentration.2 We determined the likelihood of achieving these values in a single early morning urine sample from healthy children.

Subjects, methods, and results

We collected an early morning urine sample from 322 healthy children and adolescents (age range 3 years 11 months to 18 years 8 months, median 9.8 years; 170 males) from five local schools. The study was approved by the ethics and education management committees, and informed written consent was obtained from subjects or their parent or guardian. Subjects were asked to avoid drinking after going to bed, unless they found it uncomfortable to do so. No one had a personal or family history of renal or urinary tract disease or was receiving potentially nephrotoxic drugs. pH was measured with a digital pH meter and osmolality with a freezing point osmometer on the morning of collection. The cumulative distribution function was calculated for each value to determine the probability of observing a value above or below a specified point. Smoothing was performed by means of Gaussian kernels.3 Reference ranges were determined from the resultant density estimates as the values between the 2.5th and the 97.5th centiles.

One child admitted to drinking after bedtime and eight to collecting the second urine sample of the day after breakfast; 313 (98%) subjects collected the first early morning sample. Osmolality was measured in all 322 samples and pH in 318. Median pH was 6.0 (range 4.8-7.7). The reference range was 5.16-7.07; only 12.8% of children had a value of 5.4 or less. Median osmolality was 845 mmol/kg (range 275-1344). Males had significantly higher osmolalities than females (median 896 v 781 mmol/kg; Mann-Whitney U test, P<0.005); no sex difference was observed for pH. The reference range of urine osmolality was 417-1218 mmol/kg for the males and 329-1194 mmol/kg for the females; only 82.2% of males and 74.8% of females had an osmolality of 600 mmol/kg or more.

Comment

To our knowledge, this is the first study to establish reference ranges for osmolality and pH of an early morning urine sample in healthy children and adolescents after an overnight thirst, although these variables are widely measured by paediatricians and used in clinical assessment. The only other study of urinary concentration in normal children used a longer period of formal fluid deprivation.4 So far as we know, urinary acidification has never been studied in this way.

Our study clearly shows that not to have a urinary pH of 5.4 or less, or an osmolality of 600 mmol/kg or more, in a single early morning urine sample cannot be taken as evidence of impairment of urinary acidification or concentration because the specificity of these measures is low. Only about one child in eight will have a pH of 5.4 or less, making this an almost useless screening test. Although osmolality is more useful for evaluating urine concentration, only around four children out of five will have an osmolality of 600 mmol/kg or more. The use of early morning urine pH and osmolality is attractive in its simplicity, but failure to appreciate the limitations of these tests may lead to an incorrect diagnosis of distal tubular impairment.

We thank the department of clinical biochemistry, Royal Victoria Infirmary, Newcastle upon Tyne for help with the investigations; Dr M M Reid for his constructive comments; the Assistant Director of Education (Schools Management), Newcastle upon Tyne, and the schools and teachers for their willing cooperation; and the children and adolescents who provided samples.

Footnotes

  • Funding Medical Research Council (RS). Additional funds came from the Special Trustees of Newcastle Health Authority and the North of England Children's Cancer Research Fund.

  • Conflict of interest None.

References

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