Endocrine Abstracts

Introduction: Hyponatremia (serum sodium ≤135 mmol/l), the most common electrolyte disorder encountered, has been associated with increased mortality in patients with particularly cancer, heart failure, chronic kidney and liver disease. However, evidence of the clinical implications in broader populations is scarce, and uncertain due to confounding from preexisting disease. We aimed to examine the association between admission-hyponatremia and 30-day mortality in acute non-surgical patients.

Method: We conducted a population-based cohort study in North and Central Denmark Regions, comprising approximately 1.8 million inhabitants. We identified all patients acutely admitted to non-surgical departments from January 1, 2000 to December 31, 2009, for whom serum sodium was measured on the day of admission using individual level linkage of the Danish National Patients Registry, the Danish Civil Registration System and the Clinical Laboratory Information System. Admission-hyponatremia was categorized as mild (130–135 mmol/l), moderate (125–129.9 mmol/l) and severe (<125 mmol/l). Thirty-day mortality for normonatremia and levels of admission-hyponatremia were estimated using the Kaplan–Meier method. Mortality rate ratios (MRRs) were estimated using a Cox regression model, adjusting for sex, age and comorbidity level.

Results: We identified 302,311 acute non-surgical patients, with a sodium measurement on the first day of admission. Admission-hyponatremia was present in 70140 patients (prevalence=23%). Hyponatremic patients were older and had higher comorbidity levels than normonatremic patients. Cumulative 30-day mortality for patients with admission-hyponatremia was 8.8% compared with 3.9% in normonatremic patients. Mortality was increased throughout the period. Adjusted MRR was 1.78 (95% CI: 1.72–1.84) for any hyponatremia compared with normonatremia. Adjusted MRR for mild, moderate and severe hyponatremia compared with normonatremia was 1.59 (95% CI: 1.54–1.65), 2.32 (95% CI: 2.19–2.46) and 2.40 (95% CI 2.23–2.60) respectively.

Discussion: Admission-hyponatremia was associated with increased risk of death for all levels of hyponatremia, even after adjusting for higher comorbidity level in hyponatremic patients. The risk increased with decreasing sodium levels.