Social Impact and Economic Burden of Uncoded Hyponatremia in Elderly - A Cost-of-Illness Study

Hyponatremia remains one of the most prevalent electrolyte disorders among geriatric inpatients but is frequently under-recognized in clinical coding. This cost-of-illness (COI) study aimed to (1) estimate the corrected prevalence of uncoded hyponatremia (ucHn) by integrating laboratory and administrative data, and (2) quantify its incremental social and economic burden on elderly patients.

A retrospective COI analysis covered 72,730 inpatient cases ≥70 years (2016-2024) in a Swiss hospital network. Hyponatremia was classified as coded (cHn; ICD-10 E87.1) or uncoded (ucHn; Na <135 mmol/L). Incremental costs were estimated via generalized linear models and probabilistic sensitivity analysis.

Among 13,657 patients with hyponatremia (18.7%), only 2,070 (2.8%) were coded. Prevalences were as follows: coded 2.8%, hidden15.9%, true prevalence 18.7%. ucHn was more prevalent in men (p<0.0001), presented with more mild cases (83.1/34.9%, p<0.0001), more incident cases (14.3/4.7%, p<0.0001), lower chronic hyponatremia (20.8/39.6% p<0.0001), significantly more frequent heart failure and lung cancer (p<0.0001), had higher asset cost (2,463 vs 1,654, p<0.0001), lower contribution margin 1 and 2 (p<0.0001) and a markedly higher 30-day mortality (46% vs 7%, p<0.0001). Mean length of stay (LOS) was 9.9 days vs 7.4 days (p<0.0001). ucHn generated incremental costs of CHF per case with a total systemic burden (2016-2024) amounted to ≈ 355 to 473 MCHF. Losses of autonomy, mobility, cognitive control and life years underline the impressive social impact for nearly every ucHn case.

For the first time, the detected ucHn is evaluated. It presents a substantial social and economic burden with a previously unquantified excess mortality associated with uncoded hyponatremia. ucHn is widely underestimated in administrative hospital data. This COI study supports policy measures to improve documentation and awareness of hyponatremia in elderly patients with the aim of reducing its social impact.