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Relationship between timed and spot urine collections for measuring phosphate excretion

  • Nephrology - Original Paper
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Abstract

Background

Twenty-four hour urinary phosphate excretion (UPE) reflects intestinal phosphate absorption in steady state and may be more informative than serum phosphate (sPi) when assessing phosphate homoeostasis clinically. Timed urine collections are cumbersome and prone to collection errors. Spot urine phosphate/creatinine ratio (uPiCr) may be a useful, simple surrogate for 24-h UPE, but requires further validation. This study aimed to determine the relationship between uPiCr and 24-h UPE.

Methods

This single-centre cross-sectional study examined contemporaneous serum, spot urine and 24-h urine. Serum biochemistry was analysed. Urine phosphate concentration (uPi) and creatinine concentration (uCr) were measured in spot and 24-h urine collections. Spearman’s rank correlation coefficients and Bland–Altman plots were used to assess agreement between spot uPiCr and UPE. Backward multivariate analysis was undertaken for UPE prediction.

Results

One hundred and sixteen participants (77 with kidney disease and 39 healthy volunteers) were studied. Seventy-four (63.8 %) were male. Median (IQR) age was 61(49–71) years. Median (IQR) spot uPiCr and total UPE were 1.7 (1.3–2.2) mmol/mmol and 25.8 (19.9–35.0) mmol/d, respectively. There was no correlation between spot uPiCr and 24-h UPE (R = 0.064, P = 0.51) but spot uPi significantly correlated with 24-h UPE (R = 0.385, P < 0.001). Although spot and 24-h measures of phosphate handling correlated (all P < 0.001), Bland–Altman analysis revealed bias between collection techniques. UPE prediction model using the independent variables of eGFR, sPi, albumin and spot uPi provided R 2 = 0.443.

Conclusion

No direct correlation was noted between spot uPiCr and 24-h UPE. Normalization of uPi to uCr on spot urine samples may be inappropriate when evaluating urinary phosphate excretion in adults and thus, a spot uPiCr is not a suitable surrogate for measuring UPE. A UPE prediction model utilising spot urine biochemistry cannot be advocated at present.

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Acknowledgments

SJT is a current recipient of a National Health and Medical Research Council (NHMRC) Postgraduate Research Scholarship. NDT is supported by a Jacquot Foundation Research Establishment Award. The contents of this article are solely the views of the individual authors and do not reflect the views of NHMRC or the Jacquot Foundation.

Funding

This study was supported by a Jacquot Foundation Research Establishment Award (see also Acknowledgements).

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Authors and Affiliations

  1. Department of Nephrology, The Royal Melbourne Hospital, Parkville, VIC, 3052, Australia

    Sven-Jean Tan, Edward R. Smith, Michael M. X. Cai, Stephen G. Holt, Tim D. Hewitson & Nigel D. Toussaint

  2. Department of Medicine (RMH), The University of Melbourne, Melbourne, Australia

    Sven-Jean Tan, Michael M. X. Cai, Stephen G. Holt, Tim D. Hewitson & Nigel D. Toussaint

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  1. Sven-Jean Tan
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  2. Edward R. Smith
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  6. Nigel D. Toussaint
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Corresponding author

Correspondence to Sven-Jean Tan.

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Conflict of interest

SJT has received speaking honoraria from Shire. ERS has received research funding from Amgen and Baxter, honoraria from Shire, and served as a consultant for Vifor Pharma. SGH has received research funding or honoraria from Amgen, Baxter, Gilead, Novartis and Shire. NDT has received consultancy fees, honoraria and research funding from Amgen and Shire Pharmaceuticals. MMXC and TDH have no conflict of interests to declare.

Ethical approval

The study presented within this manuscript was approved by a local human research ethics committee and conducted in accordance with the Declaration of Helsinki.

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Tan, SJ., Smith, E.R., Cai, M.M.X. et al. Relationship between timed and spot urine collections for measuring phosphate excretion. Int Urol Nephrol 48, 115–124 (2016). https://doi.org/10.1007/s11255-015-1149-z

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  • DOI: https://doi.org/10.1007/s11255-015-1149-z

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