'Normal' serum sodium concentration among inpatients over 65 admitted to hospital: an observational study

The usual reference range of serum sodium (typically 135-145 mmol/l) is derived from healthy populations but may not apply to hospital patients.

To describe the range of serum sodium in inpatients and outpatients of both sexes at different ages. To ascertain correlates of serum sodium among older inpatients. To describe the association between sodium and mortality.

We used routine hospital data on serum sodium in inpatients admitted between 1 January 2011 and 31 December 2014 and patients attending outpatient or community clinics with no record of admission to the same hospitals in the same period. We investigated the distribution of sodium values within these groups and explored the relationship between serum sodium and death using logistic regression.

Levels among hospital inpatients were significantly lower than in outpatients of the same age. Emergency admissions had lower levels and those admitted from care homes higher levels, risk of inhospital death began to rise at 140 mmol/L, well within the 'normal' range. Patients with a serum sodium concentration of 145 mmol/L on admission have a risk of inhospital death 3.7 times higher than that of a patient with a concentration of 140 mmol/L.

The range for serum sodium concentration on admission in inpatients is broader and lower than the commonly accepted reference range. The risk of mortality increases at sodium concentration >139 mmol/L, well within reference range currently considered normal.