Preoperative sodium abnormalities and adverse postoperative outcomes in older surgical patients: association and incremental predictive value in the BioCog cohort.

PURPOSE: To determine whether preoperative sodium abnormalities provide incremental predictive value for estimating postoperative delirium (POD), mortality, and complications. METHODS: For this observational cohort study, we included older patients (≥ 65 years) without dementia who were scheduled for elective surgery with an anticipated duration of ≥ 60 min. Routinely obtained sodium values below 135 mmol/L defined hyponatremia and that above 145 mmol/L hypernatremia. We assessed our primary outcome, postoperative delirium, by using a set of four validated tests. Our secondary, exploratory outcomes were postoperative complications, and in-hospital as well as 90-day mortality. For each outcome, we calculated multivariable prediction models and determined the change of the area under the receiver-operating curve (AUROC) and Brier scores after adding abnormal sodium as variable. RESULTS: Presurgical sodium levels were available for 618 patients with a median age of 72 (Interquartile range 7) and 286 (46.3%) female patients. Thirty-one patients (5.0%) had values indicating hyponatremia (range 125-134 mmol/L) and 16(2.6%) hypernatremia (range 146-150 mmol/L). Patients with abnormal sodium levels had higher incidences for each outcome: POD [normal sodium 119(20.8%) vs. abnormal sodium 18(38.3%)], in-hospital mortality [9(1.6%) vs. 8(17.0%)], the 90-day mortality [16(2.8%) vs. 13(27.7%)], and more severe complications (Clavien-Dindo-II and more) [219(38.4%) vs. 30(65.2%)]. After adjustment for established clinical risk factors, the addition of dysnatremia resulted in only minimal improvements in discrimination for postoperative delirium (AUROC 0.71 to 0.72; bootstrap-corrected difference 0.0045, 95% CI - 0.0027 to 0.0199) and postoperative complications (AUROC 0.71 to 0.72; difference 0.004, 95% CI - 0.0011 to 0.0124). For in-hospital mortality, the inclusion of dysnatremia increased the optimism-corrected AUROC from 0.864 to 0.889 (difference 0.023, 95% CI 0.0046-0.0435). Similarly, for 90-day mortality, addition of dysnatremia increased the bootstrap-validated C-index from 0.91 to 0.93, corresponding to a median increase of 0.021 (95% CI 0.001 to 0.057). However, Brier scores only showed marginal differences for each outcome. CONCLUSIONS: While preoperative sodium disorders are associated with postoperative risk, it added limited incremental value for predicting POD and postoperative complications, offering little clinical impact. In exploratory analysis, sodium abnormalities slightly improved risk discrimination of 90-day mortality beyond established risk factors, which warrants further study. CLINICAL TRIAL NUMBER: NCT02265263 ( https://clinicaltrials.gov/ct2/show/results/NCT02265263 ).