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Kidney Week

Abstract: PO1160

Estimating 24-Hour Urinary Excretion Using Spot Urine Measurements in Kidney Stone Formers

Session Information

Category: Fluid, Electrolyte, and Acid-Base Disorders

  • 902 Fluid, Electrolyte, and Acid-Base Disorders: Clinical


  • Ferraro, Pietro Manuel, Università Cattolica del Sacro Cuore, Rome, Rome, Italy
  • Lopez Osma, Fernando, Maine Medical Center, Portland, Portland, Oregon, United States
  • Barbarini, Silvia, Università Cattolica del Sacro Cuore, Rome, Rome, Italy
  • Curhan, Gary C., Brigham and Women's Hospital Channing Division of Network Medicine, Boston, Massachusetts, United States
  • Taylor, Eric N., VA Maine Healthcare System, Augusta, Maine, United States

One limitation of the use of the 24-h collection, a key element in the management of kidney stone (KS) disease, is impracticality. To overcome this limitation, we analyzed the performance of spot urine measurements to estimate 24-h excretion in patients with KS.


74 adult KS patients from two centres were instructed to perform a 24-h urine collection. A sample of the last micturition (fasting, upon awakening) was sent for spot urine analysis. Twenty patients were asked to collect two additional spot urine samples, one before dinner (pre-prandial) and the other after dinner (post-prandial). Urinary concentrations of creatinine, calcium, oxalate, uric acid, citrate and magnesium were measured in the 24-h and each of the spot urine samples. Three approaches were used to estimate 24-h urinary excretion, multiplying the ratio of the spot urinary analyte to creatinine concentration by 1) measured 24-h urinary creatinine excretion [“Prediction #1”], 2) estimated 24-h urinary creatinine excretion [“Prediction #2”], or 3) assumed 1 gram 24-h urinary creatinine excretion [“Prediction #3”]. For each parameter we computed Lin’s concordance correlation coefficients (CCCs), Bland-Altman plots, and 95% limits of agreement.


The performance of estimates obtained with Prediction #1 and Prediction #2 was similar for all parameters, except for citrate and uric acid for which Prediction #2 performed significantly worse. Both estimation approaches performed moderately well: citrate CCC 0.82 (95% CI 0.75, 0.90), oxalate 0.66 (0.55, 0.78), magnesium 0.66 (0.54, 0.77), calcium 0.63 (0.50, 0.75), uric acid 0.52 (0.36, 0.68). The performance of Prediction #3 was consistently worse. Post-prandial samples tended to perform numerically worse compared with fasting morning and pre-prandial samples except for uric acid.


Utilizing measured or estimated 24-h creatinine substantially increases the utility of spot urine samples in estimating 24-h excretion of urinary analytes in KS formers.