The prognostic importance of serum sodium levels at hospital discharge and one‐year mortality among hospitalized patients

Background The optimal range of serum sodium at hospital discharge is unclear. Our objective was to assess the one‐year mortality based on discharge serum sodium in hospitalized patients. Methods We analyzed a cohort of hospitalized adult patients between 2011 and 2013 who survived hospital admission at a tertiary referral hospital. We categorized discharge serum sodium into five groups; ≤132, 133‐137, 138‐142, 143‐147, and ≥148 mEq/L. We assessed one‐year mortality risk after hospital discharge based on discharge serum sodium, using discharge sodium of 138‐142 mEq/L as the reference group. Results Of 55 901 eligible patients, 4.9%, 29.8%, 56.1%, 8.9%, 0.3% had serum sodium of ≤132, 133‐137, 138‐142, 143‐147, and ≥148 mEq/L, respectively. We observed a U‐shaped association between discharge serum sodium and one‐year mortality, with nadir mortality in discharge serum sodium of 138‐142 mEq/L. When adjusting for potential confounders, including admission serum sodium, one‐year mortality was significantly higher in both discharge serum sodium ≤137 and ≥143 mEq/L, compared with discharge serum sodium of 138‐142 mEq/L. The mortality risk was the most prominent in elevated discharge serum sodium of ≥148 mEq/L (HR 3.86; 95% CI 3.05‐4.88), exceeding the risk associated with low discharge serum sodium of ≤132 mEq/L (HR 1.43; 95% CI 1.30‐1.57). Conclusion The optimal range of serum sodium at discharge was 138‐142 mEq/L. Both hypernatremia and hyponatremia at discharge were associated with higher one‐year mortality. The impact on higher one‐year mortality was more prominent in hypernatremia than hyponatremia.