Urinary sodium, baseline loop diuretic dose, and short-term outcomes in ambulatory worsening heart failure: The NaU/F Index.

Background

Worsening heart failure (WHF) carries high short-term morbidity and mortality. Spot urinary sodium (NaU) is increasingly used to assess diuretic response but does not account for chronic loop diuretic exposure.

Objectives

To determine whether a urinary sodium-to-furosemide index (NaU/F) improves short-term risk stratification compared with isolated NaU in ambulatory WHF

Methods

Retrospective analysis of a prospective cohort of consecutive adults treated in a specialized heart failure day-hospital program (August 2023-April 2025). Spot NaU was measured 2 hours after intravenous loop diuretic administration. NaU/F was calculated as urinary sodium divided by baseline oral furosemide dose (40-mg equivalents). PRIMARY ENDPOINT 30-day composite of HF rehospitalization, unplanned outpatient visit, or all-cause mortality.

Results

130 patients were included (mean age 77.8 ± 10.8 years; 74.6% male; LVEF 42.2 ± 18.2%). The primary endpoint occurred in 10.9%. NaU <70 mmol/L showed modest prognostic performance (OR 2.70, 95% CI 0.85-8.57; p=0.08; AUC 0.67). NaU/F <26 mmol/40 mg was associated with higher event rates (20% vs 5%; OR 4.69, 95% CI 1.38-15.90; p=0.008) and better discrimination (AUC 0.77, 95% CI 0.67-0.86), remaining independently associated with the endpoint (p=0.02).

Conclusion

In ambulatory WHF, NaU/F outperformed isolated urinary sodium for short-term risk stratification. Contextualizing natriuretic response to chronic loop diuretic exposure may refine prognostic assessment and warrants prospective validation.