paired plasma and urine osmolality

Abstract

Hyponatraemia is defined as a serum sodium concentration of 135 mmol/L or less. It is the most common electrolyte abnormality, and can be associated with significant morbidity and mortality.1 There are numerous causes and a systematic approach to the initial investigation and management is required. Clinical assessment should start with evaluation of the patient’s degree of hydration, with the measurements of serum and urine osmolality and urinary sodium concentration being the key biochemical investigations. A recent article in this journal highlighted the challenges associated with acute symptomatic hyponatraemia; this article concentrates on the more common scenario of the patient with asymptomatic hyponatraemia.