Purpose: 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.
Patients and methods: 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.
Results: 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.
Conclusion: 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.
Keywords: DRG; PSA; bottom-up and top-down calculation; diagnosis-related group; financial losses; geriatric electrolyte shift; mortality; probabilistic sensitivity analysis.
Why was this study done?: Hyponatremia means that the concentration of sodium in the blood is too low, most often because of an excess of water rather than a true lack of sodium. This condition is common in older hospital patients and can cause serious problems like falls, confusion and even death. However, doctors often fail to record this condition in medical files. The researchers wanted to find out how many cases are missed and what this means for patients and hospitals.
What did the researchers do and find?: The team analyzed medical records from over 72,000 patients aged 70 and older at a Swiss hospital between 2016 and 2024. They combined blood test results with hospital records to identify both recorded and unrecorded cases of hyponatremia.Their findings were striking: while blood tests showed that 13,657 patients (18.7%) had hyponatremia, only 2,070 cases (2.8%) were recorded in their medical files. This means 85% of cases went undocumented.Patients with unrecorded hyponatremia fared worse than those whose condition was documented. They stayed in hospital longer (9.9 days versus 7.4 days) and their death rate within 30 days was dramatically higher (46% versus 7%). Many lost their independence, mobility, and mental sharpness.The economic impact was substantial, with unrecorded cases costing Swiss hospitals between 355 and 473 million francs over nine years.
What do these results mean?: This study reveals that unrecorded hyponatremia creates a hidden burden on older patients and healthcare systems. Better awareness and documentation could help doctors recognize and treat this condition earlier, potentially preventing serious complications and saving lives. The findings support the need for improved recording practices in hospitals to ensure patients receive appropriate care.
© 2026 Weinrebe et al.