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. 2025 Apr 10;18(5):sfaf097.
doi: 10.1093/ckj/sfaf097. eCollection 2025 May.

Estimating 24-hour urine phosphate excretion from spot urine

Yongchao Li  1   2   3 , Daniel G Fuster  4   5 , Nasser A Dhayat  6 , Harald Seeger  7   8 , Alexander Ritter  7   9 , Olivier Bonny  4   10   11 , Gregoire Wuerzner  11 , Thomas Ernandez  12 , Stephan Segerer  13 , Beat Roth  14 , Isabel Rubio-Aliaga  1   4 , Carsten A Wagner  1   4
Affiliations

Estimating 24-hour urine phosphate excretion from spot urine

Yongchao Li et al. Clin Kidney J. .

Abstract

Background: 24-hour urinary phosphate excretion (24hUrP) is indicative of intestinal phosphate absorption in steady-state conditions. Nevertheless, 24-hour urine collections are cumbersome and error-prone. Previous studies suggested that spot urine phosphate (uPi) could serve as a practical substitute to predict 24hUrP, however, these data originated only from patients with chronic kidney disease. Here, we investigated the validity of predictive equations using spot urine parameters to assess 24hUrP in a cohort with normal kidney function (eGFR >60 ml/min per 1.73 m2) including 761 kidney stone patients and 207 non-kidney stone formers as assessed by low-dose CT scans, the Swiss Kidney Stone Cohort (SKSC).

Methods: Published equations for 24hUrP were tested in our cohort and a novel predictive equation was developed. Pearson correlation coefficients and Bland-Altman plots were used to assess the relationship between spot uPi and spot urine creatinine (uCr) and 24hUrP. Additionally, forward multivariate analysis was performed to predict uPi excretion.

Results: Previously published equations provided less accurate prediction of 24hUrP from spot urine. Log-transformed 24hUrP with log-transformed spot uPi and creatinine yielded the best model fit. In addition, inclusion of age, sex, and BMI significantly improved prediction of 24hUrP. Compared with spot uPi and uCr alone (r 2 = 0.0561, P < .001) the new equation predicted 24hUrP (r 2 = 0.1820, P < .001) more accurately.

Conclusions: Here, we present a new equation for predicting 24hUrP from spot urine samples of individuals with normal kidney function. This model has a moderate ability to explain 24hUrP variance but has the strength to use only parameters routinely collected in clinical settings such as spot urinary phosphate and creatinine, sex, BMI, and age.

Keywords: 24-hour urine phosphate; kidney disease; predictive equation; spot urine; the Swiss Kidney Stone Cohort.

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Conflict of interest statement

C.A.W. reports honoraria from Kyowa Kirin and Medice, and scientific collaborations with Chugai and Bayer AG. D.G.F. served as a consultant for Otsuka, Alnylam, Boehringer Ingelheim, and Kyowa Kirin, and received unrestricted research grants from Otsuka, Boehringer Ingelheim, and CSL Vifor. G.W.R. reports honoraria from Servier, AstraZeneca, Medtronic, Recor, and Bayer. A.R. received speaker fees from Alnylam, CSL Vifor, Boehringer Ingelheim, AstraZeneca, and Forum für medizinische Fortbildung (FOMF) and support for travel expenses and attending meetings by Astellas, Boehringer Ingelheim, and Salmon Pharma outside this work. S.T.S. reports honoraria from Fresenius and Baxter (speaker and traveling fee), shareholder of Novartis, Sandoz, and Calciscon. B.R., N.A.D., H.S., O.B., T.E., I.R.A., and Y.C.L. declare no conflicts of interest.

Figures

Figure 1:
Figure 1:
Flow chart of the study population derived from the SKSC.
Figure 2:
Figure 2:
Correlations of measured versus predicted 24-h urine phosphate for linear combination of spot urine phosphate and urine creatinine concentrations. (a, d, g) Scatterplots of measured 24-h phosphate excretion compared with 24-h phosphate excretion estimated from urine spot samples. (b, e, h) The nonlinear relationship between predicted and observed 24-h urine phosphate concentration. A smooth curve fitting between variables is represented by the solid black line. The 95% confidence interval of the fit is indicated by the gray line. (c, f, i) Bland–Altman plots of the average of 24-h phosphate excretion versus the difference between the estimated 24-h phosphate excretion from urine spot samples and the measured 24-h phosphate excretion. The x-axis indicates the mean of measured and estimated 24-h phosphate excretion; the y-axis indicates the mean difference between estimated and measured 24-h phosphate excretion; the continuous lines indicate the mean differences; and the dashed lines indicate the 95% LOA of the mean difference (mean ± 1.96 SD).
Figure 3:
Figure 3:
Correlations of measured versus predicted 24-h urine phosphate for spot urine phosphate-to-creatinine ratio using no transformation, partial or complete log transformation. (a, d, g) Scatterplots of measured 24-h phosphate excretion compared with 24-h phosphate excretion estimated from urine spot samples. (b, e, h) The nonlinear relationship between predicted and observed 24-h urine phosphate concentration. A smooth curve fitting between variables is represented by the solid black line. The 95% confidence interval of the fit is indicated by the gray line. (c, f, i) Bland–Altman plots of the average of 24-h phosphate excretion versus the difference between the estimated 24-h phosphate excretion from urine spot samples and the measured 24-h phosphate excretion. The x-axis indicates the mean of measured and estimated 24-h phosphate excretion; the y-axis indicates the mean difference between estimated and measured 24-h phosphate excretion; the continuous lines indicate the mean differences; and the dashed lines indicate the 95% LOA of the mean difference (mean ± 1.96 SD).
Figure 4:
Figure 4:
Correlation of predicted and measured 24 h urine phosphate excretion using the developed formula. (a) Scatter plots of measured 24-h phosphate excretion compared with 24-h phosphate excretion estimated from urine spot samples. (b) The nonlinear relationship between predicted and observed 24-h urine phosphate concentration. A smooth curve fitting between variables is represented by the solid black line. The 95% confidence interval of the fit is indicated by the gray line. (c) Bland–Altman plots of the average of 24-h phosphate excretion versus the difference between the estimated 24-h phosphate excretion from urine spot samples and the measured 24-h phosphate excretion.
Figure 5:
Figure 5:
The nonlinear relationship between predicted and observed 24-h urine phosphate amount in the groups of KSF (dashed line) and NKSF (solid line).
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