A case-control study of hyponatraemia as an independent risk factor for inpatient mortality

Hyponatraemia is strongly associated with increased inpatient mortality, but it is unknown whether hyponatraemia per se contributes to excess mortality. Our hypothesis was that if hyponatraemic patients had significantly greater mortality compared with controls despite no difference with regard to gender, age, comorbidities and type of primary pathology, this would incriminate hyponatraemia as an independent predictor of mortality.

Single-centre, case-control study.

Cases (N = 139) were hospitalized patients with serum Na ≤ 128 mmol/l over 3 months. Controls were 254 age- and gender-matched patients residing in the same hospital ward with serum Na > 128 mmol/l.

Data were collected about age, gender, comorbidities, drug history, serum creatinine, intensive care unit (ICU) admission and length of hospitalization. The main outcome measure was inpatient mortality.

Hyponatraemic patients had an inpatient mortality rate of 17·3% and were more than three times more likely to die during their hospital stay compared with controls (OR 3·33, 95% CI 1·68-6·58, P < 0·01) despite no statistically significant difference with respect to age, gender, comorbidities, use of common drugs, serum creatinine, ICU admission rate and length of hospitalization. Comparison of cases with the normonatraemic subgroup of controls demonstrated that cases were almost 12 times more likely to die during admission than normonatraemic controls (OR 11·89, 95% CI 2·75-51·51, P < 0·01).

This study showed that hyponatraemia is an independent predictor of mortality, and hyponatraemia per se is likely to contribute to excess mortality. Further studies are needed to examine whether correction of hyponatraemia can reduce mortality.