Development and validation of a prediction model for estimating 24-Hour Urinary Sodium Excretion using spot urine samples in adult patients.

The measurement of urinary sodium excretion provides valuable information about an individual's sodium balance and can help in the management of various medical conditions. However, the collection of 24-hour urine samples is subject to errors. Spot urine sodium (uNa) measurements are a reliable alternative to 24-hour urine collections for estimating urinary sodium excretion.

Objective: To assess whether 24-h urinary sodium excretion (24uNa) can be estimated from spot samples in adult patients who attend hospital clinics.

Design: Methods: A cross-sectional study with a development (284 patients) and a validation cohort (229 patients) was conducted at our hospital. A multivariate linear regression model was built which was compared with former models. Concordance analyses and comparison of the ability to correctly classify each patient against a prespecified uNa cut-off value of 130 mmol/24h were performed, assessed by the C-statistic.

Results: The model was well calibrated (slope [95%CI] in internal validation: 0.965 [0.947-0.987], showing good discrimination, and performed robustly in an external validation cohort (slope: 0.811 [0.675-0.946]). The mean bias between the measured and the estimated 24uNa by NaRYC was 24.85 mmol/24h [17.06-32.63]. The NaRYC had the highest values of Pearson coefficient (0.613 p<0.0001), accuracy (P30): 56.8%, and AUC-ROC: 0.822 [0.766-0.869] as compared to other seven equations.

Conclusion: Although the mean bias of the results is quite acceptable, the variability observed in the 95%CI makes not recommend the general use of a spot as a substitute of the 24-hour urine in order to estimate the total urine excretion of Na in a single subject basis.