The goal of this study is to find differences between patients who do and do not develop ICU-acquired hypernatremia (IAH). Therefore extended sodium and fluid balances will be performed. Also a couple of other factors that possibly contribute to the development of IAH and/or could give clues about the mechanisms in the development of IAH will be investigated.

ICU-acquired hypernatremia (IAH) is common in critically ill patients, but it's aetiology is still not fully understood. Sodium overload and inadequate fluid administration are generally considered important factors, but a previous retrospective study did not reveal differences in sodium or fluid balances between patients who did and did not develop IAH. Because this was retrospective these balances will be prospectively studied in this study to be more sure about the importances of these balances in the development of IAH. Previous studies indicated that (a lot) more factors contribute to the development of IAH. In this study these factors will be investigated, hoping to find differences between patients who do and do not develop IAH.

Inclusion Criteria:

• Predicted duration of ICU-admission >48 hours

Exclusion Criteria:

• Renal replacement therapy
• Reason for admission necessitates actively elevation of serum sodium concentration
• Electrolyte disturbance as reason for admission
• ICU re-admission within 30 days
• Referral from another ICU
• Serum sodium concentration on admission 143mmol/l or above

All unplanned ICU-admissions

Collected data will be available through an online portal / storage website as soon as the manuscript was published. From that moment data can be obtained after a request via the online portal / storage website.