Hyponatraemia is essentially a laboratory diagnosis, defined as a serum sodium concentration of <135 mmol/L. History and physical examination establish volume status and are used to determine if the patient is hypovolaemic, hypervolaemic, or euvolaemic. A thorough review of any underlying medical conditions and medications should be undertaken. The cause of the hyponatraemia is often apparent from the history and examination; however, some causes can only be identified with appropriate investigations. Patients with asymptomatic, mild hyponatraemia (130-135 mmol/L) may be managed initially in a community setting. Initial assessment should also include glycaemic status, to establish whether there is a hyperglycaemic-induced hyponatraemia, and the exclusion of pseudohyponatraemia caused by excessive plasma lipids or proteins. Patients with onset of hyponatraemia in <48 hours, or those with symptoms, require urgent assessment.
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