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Abstract: PO1110

Validity of a Simple Equation to Estimate Urine Output in Outpatients with Suspected Nephrolithiasis

Session Information

Category: Fluid, Electrolyte, and Acid-Base Disorders

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


  • Webster, Luke, University of California San Diego, La Jolla, California, United States
  • Bullen, Alexander, University of California San Diego, La Jolla, California, United States
  • Asplin, John R., LithoLink Corp, Chicago, Illinois, United States
  • Shlipak, Michael, University of California San Francisco, San Francisco, California, United States
  • Bechis, Seth, University of California San Diego, La Jolla, California, United States
  • Monga, Manoj, University of California San Diego, La Jolla, California, United States
  • Ix, Joachim H., VA San Diego Healthcare System, San Diego, California, United States

We previously developed and validated an equation to estimate urine volume from spot urine creatinine, demographic variables, and body weight in patients with kidney disease. We hypothesized this equation could accurately estimate daily urine output in outpatients with nephrolithiasis.


Among persons submitting specimens to Litholink Laboratories between May 2013 and January 2016, we identified 19,884 individuals who had two 24 hour urine collections on consecutive days, with creatinine excretion rates ±10%. We used Pearson correlations and Bland-Altman analysis to evaluate equation estimated with measured urine volume on the first of the two 24 hour urine collections. We also tested the equation’s test characteristics to accurately identify producers >2L/day; a guideline directed urine output goal among stone formers.


Estimated and measured urine volumes were strongly correlated (r=0.76, p < 0.001). The estimated urine flow rate was 195 ml/day (8.7%) higher than measured. Overall, 78% of individuals had an estimated urine volume within 30% of measured. Performance was similar in men and women, across strata of body weight, in those with and without CKD, and when evaluating the second day measured urine volume. An estimated urine volume ≥ 2L/day had 86% sensitivity, 83% specificity, 76% PPV and 91% NPV for identifying individuals with measured urine volumes > 2L/day (Figure 1).


A simple equation using urine creatinine, demographics and body weight can predict urine flow rate among patients with nephrolithiasis. The equation may provide a tool to enhance diagnostic accuracy in the urinary risk factors for stones, and to identify patients among whom efforts to increase hydration could be targeted to diminish risk of stone recurrence.


  • NIDDK Support