title: Relationship between timed and spot urine collections for measuring phosphate excretion.
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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Nephrology, The Royal Melbourne Hospital, Parkville, VIC, 3052, Australia. Jean.Tan@mh.org.au.|Department of Medicine (RMH), The University of Melbourne, Melbourne, Australia. Jean.Tan@mh.org.au.
authors.forename: Sven-Jean
authors.initials: SJ
authors.lastname: Tan
authors.affiliation: Department of Nephrology, The Royal Melbourne Hospital, Parkville, VIC, 3052, Australia.
authors.forename: Edward R
authors.initials: ER
authors.lastname: Smith
authors.affiliation: Department of Nephrology, The Royal Melbourne Hospital, Parkville, VIC, 3052, Australia.|Department of Medicine (RMH), The University of Melbourne, Melbourne, Australia.
authors.forename: Michael M X
authors.initials: MM
authors.lastname: Cai
authors.affiliation: Department of Nephrology, The Royal Melbourne Hospital, Parkville, VIC, 3052, Australia.|Department of Medicine (RMH), The University of Melbourne, Melbourne, Australia.
authors.forename: Stephen G
authors.initials: SG
authors.lastname: Holt
authors.affiliation: Department of Nephrology, The Royal Melbourne Hospital, Parkville, VIC, 3052, Australia.|Department of Medicine (RMH), The University of Melbourne, Melbourne, Australia.
authors.forename: Tim D
authors.initials: TD
authors.lastname: Hewitson
authors.affiliation: Department of Nephrology, The Royal Melbourne Hospital, Parkville, VIC, 3052, Australia.|Department of Medicine (RMH), The University of Melbourne, Melbourne, Australia.
authors.forename: Nigel D
authors.initials: ND
authors.lastname: Toussaint
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D016022:Case-Control Studies; D003404:Creatinine / Q000652:urine*; D003430:Cross-Sectional Studies; D006801:Humans; D007674:Kidney Diseases / Q000652:urine*; D008875:Middle Aged; D010710:Phosphates / Q000652:urine*; D059349:Urine Specimen Collection / Q000379:methods*
chemical_list: D010710:Phosphates; D003404:Creatinine
keywords: Chronic kidney disease; Phosphate homoeostasis; Urinary creatinine; Urinary phosphate; Urinary phosphate creatinine ratio; Urinary phosphate excretion