A Data-Driven Threshold for Extreme Hyponatremia in the Emergency Department and Its Association With 28-Day Mortality.

Hyponatremia is a common electrolyte disorder in the emergency department (ED), but its clinical burden is not evenly distributed across the sodium spectrum. We conducted a retrospective observational cohort study of adult ED encounters at a tertiary referral hospital between July 2019 and July 2024 to identify a data-driven threshold for extreme hyponatremia and evaluate its association with 28-day mortality. Encounters with simultaneous sodium and glucose measurements were included, and sodium values were corrected for hyperglycemia. Kernel density estimation identified a distinct local minimum defining distribution-derived extreme hyponatremia (DDEH). Propensity score matching based on age and sex was used to compare DDEH with nonextreme hyponatremia and normonatremia. Among 227,027 eligible ED patients, density modeling identified an inflection point at 110 mmol/L. Sixty-one patients (0.027%) met criteria for DDEH. In matched analyses (n = 61 per group), 28-day mortality was highest in the DDEH group (36.1%), compared with nonextreme hyponatremia (18.0%) and normonatremia (6.6%) (p < 0.001). Kaplan-Meier analysis showed significantly reduced survival in patients with DDEH (log-rank p < 0.001). A distribution-derived sodium threshold of 110 mmol/L identified a rare but high-risk phenotype of extreme hyponatremia in ED patients. This cohort-derived boundary identified a rare extreme-tail phenotype associated with systemic physiological vulnerability and increased short-term mortality risk, but external validation is required before broader clinical implementation.