Estimation of 24-hour Urine Phosphate Excretion from Spot Urine Collection: Development of a Predictive Equation

Background: The management of hyperphosphatemia in patients with moderate to severe chronic kidney disease (CKD) includes dietary phosphate restriction and/or prescription of phosphate binders. Measuring phosphate intake in CKD is important for monitoring dietary adherence and for the effectiveness of therapeutic interventions. The 24-hour urine collection is the gold standard method for determining phosphate intake; however, timed urine collections are cumbersome and prone to error. We investigated the precision and accuracy of spot urine phosphate measurements, compared to 24-hour urine phosphate (24hUrP) collection.

Study Design, Setting, And Participants: We evaluated simultaneous spot and 24hUrP measurements, collected on multiple occasions, from 143 participants in the Phosphate Normalization Trial, a randomized trial of phosphate binders versus placebo among persons with an estimated glomerular filtration rate between 20-45 mL/minute per 1.73 m2. We used residual analyses and graphical methods to model the functional relationship of spot urine phosphate and creatinine measurements with 24hUrP. We used multiple linear regression to test whether additional covariates improved model prediction, including treatment assignment, age, sex, height, weight, urine collection time, and last meal time. We internally validated results using leave-one-out cross-validation, and externally validated in an independent replication cohort.

Results: A log-log relation between the spot urine phosphate-to-creatinine ratio and 24hUrP excretion yielded the best model fit. In addition to spot urine phosphate and creatinine concentrations, inclusion of age, sex, and weight significantly improved prediction of 24hUrP. Compared with a spot urine phosphate-to-creatinine ratio alone (r2 = 0.12, P < .001), the new equation more accurately predicted 24hUrP (leave-one-out validation r2 = 0.43, P < .001, independent validation r2 = 0.39, P < .001).

Conclusion: We describe a novel equation to predict 24hUrP excretion using spot urine phosphate and creatinine, age, sex, and weight. The equation is more accurate and precise than the urine phosphate-to-creatinine ratio alone, and it provides a simple method for estimating 24hUrP excretion in patients with nondialysis-requiring CKD.