Low serum sodium levels at hospital admission: Outcomes among 2.3 million hospitalized patients

Abstract

Background: Hyponatremia is the most common electrolyte disorder among hospitalized patients. Controversies still exist over the relationship between hyponatremia and outcomes of hospitalized patients.

Methods: To analyze the association of low serum sodium levels at hospital admission with in-hospital mortality and patient disposition and to compare the distribution of the risk of death associated with hyponatremia across the lifespan of hospitalized patients, we conducted an observational study of 2.3 million patients using data extracted from the Cerner Health Facts database between 2000 and 2014. Logistic regression models were used in the analyses.

Results: At hospital admission 14.4% of hospitalized patients had serum sodium levels [Na] <135 mEq/L. In adjusted multinomial logistic regression analysis, we found that the risk of in-hospital mortality significantly increases for [Na] levels < 135 or ≥143 to ≤145 mEq/L compared to the reference interval of 140 to <143 mEq/L (p<0.001). We observed similar trends for the relationship between [Na] levels and discharge to hospice or to a nursing facility. We demonstrated that younger age groups (18 to <45, 45 to <65) had a higher risk of in-hospital mortality compared to older age groups (65 to <75, ≥75) for [Na] levels <130 mEq/L or 143 to ≤145 mEq/L (p<0.001).

Conclusions: Hyponatremia is common among hospitalized patients and is significantly associated with in-hospital mortality, discharge to hospice or to a nursing facility. The risk of death and other outcomes was more evident for [Na] <135 mEq/L. The mortality associated with low [Na] was significantly higher in younger versus older patients.

Fig 1. Flow chart of the sample selection process.

^aSerum sodium levels were corrected by adding 1.6 mEq/L for each 100 mg/dL above 100 mg/dL of the concomitantly measured serum glucose levels. ^bThis number was used to calculate the prevalence of hyponatremia (Na < 135 mEq/L) at hospital admission. ^cThe final cohort available for analysis.

Fig 2. Restricted cubic splines of the crude probability of in-hospital mortality, discharge to hospice, discharge to home, and discharge to nursing facility as a function of serum sodium levels at hospital admission.

These estimated probabilities were derived from a multinomial logistic regression model. Serum sodium levels were corrected by adding 1.6 mEq/L for each 100 mg/dL above 100 mg/dL of the concomitantly measured serum glucose levels.

Fig 3. Forest plot of the relative risk ratios (95% CI) for in-hospital mortality, discharge to hospice, or discharge to nursing facility associated with different intervals of serum sodium levels (mEq/L) at hospital admission.

The relative risk rations were derived from multinomial logistic regression models adjusted for age, gender, race, and the selected comorbidities and reasons for hospitalization. Discharge to home and serum sodium levels of (140 to <143 mEq/L) served as referent. Serum sodium levels were corrected by adding 1.6 mEq/L for each 100 mg/dL increase above 100 mg/dL of the concomitantly measured serum glucose levels. Error bars indicated 95% CI. CI = confidence interval.

Fig 4. Restricted cubic splines of the estimated probability of in-hospital mortality as a function of serum sodium levels for the different age groups.

These estimated probabilities were derived from a multinomial logistic regression models stratified by age.

Fig 5. Forest plot of the relative risk ratios (95% CI) for in-hospital mortality associated with different intervals of serum sodium levels (mEq/L) at hospital admission for the different age groups.

The relative risk ratios were derived from multinomial logistic regression models adjusted for age, gender, race, and the selected comorbidities and reasons for hospitalization. Serum sodium levels of (140 to <143 mEq/L) served as referent and each age group served as its own referent. Serum sodium levels were corrected by adding 1.6 mEq/L for each 100 mg/dL increase above 100 mg/dL of the concomitantly measured serum glucose levels. Error bars indicated 95% CI. CI = confidence interval. p<0.001 for all except for age 65 to <75 in the 135 to <138 mEq/L category (p = 0.43).