Investigative algorithms for disorders affecting plasma sodium: a narrative review

Background and Objective: The following article is part of a special series to aid the reader in diagnosing the cause of various electrolyte imbalances. By the end of the article, the reader will be able to order and interpret appropriate investigations when faced with a patient with hyponatraemia or hypernatraemia. Methods: A narrative, focused literature review was performed using PubMed, OMIM and Google during September 2021 to January 2022 to identify references published from database inception to January 2022; reference lists from these articles were also used. Language was restricted to English. Key Content and Findings: Sodium is an important electrolyte, and its blood concentration is tightly controlled by the renin angiotensin aldosterone system (RAAS) and the regulation of antidiuretic hormone (ADH) secretion. However, sodium imbalance can occur in many disease states and can lead to significant morbidity and mortality, therefore finding the primary abnormality is important. A laboratory approach to investigate the cause of hyponatraemia and hypernatraemia is presented. It should be noted that sodium results are particularly susceptible to being falsely low or high in many circumstances, and the validity of sodium results should always be questioned before clinical investigation. Conclusions: Diagnostic flow charts are presented and the limitations of the laboratory tests discussed. The algorithms, by focusing on the approach to the investigation of hyponatraemia and hypernatraemia, should support healthcare professionals to efficiently and rapidly diagnose most causes of abnormal sodium states.