Hyponatraemia (serum sodium < 135 mmol/L)

• No tests in the community: refer to the emergency department (or same-day hospital opinion).
• Vomiting, confusion, drowsiness, seizures, or serum sodium < 125 mmol/L, or a fall that developed in less than 48 hours.

Laboratory tests

• Serum electrolytes (repeat serum sodium, serum potassium)
• Plasma glucose (on the same sample)
• Measured plasma osmolality
• Total serum protein
• Serum creatinine with eGFR
• TSH
• 8 am serum cortisol
• Serum urate (if SIADH suspected)if needednon-French guidelinePicture of SIADH (patient without oedema or dehydration, urinary sodium > 30 mmol/L, no diuretic): a serum urate < 0.24 mmol/L is a supporting criterion ^[1].
• Lipid profileif needednon-French guidelineLow serum sodium with normal plasma osmolality (false hyponatraemia): hypertriglyceridaemia, hypercholesterolaemia or hyperproteinaemia (monoclonal gammopathy, intravenous immunoglobulins) ^[1].
• protein electrophoresisif needednon-French guidelineLow serum sodium with normal plasma osmolality (false hyponatraemia): hypertriglyceridaemia, hypercholesterolaemia or hyperproteinaemia (monoclonal gammopathy, intravenous immunoglobulins) ^[1].

Urine tests (sample collected at the same time as the blood)

• Spot urine osmolality
• Spot urinary sodium
• Collect blood and urine the same day, before any treatment (salt intake, fluid restriction); note current diuretics on the request form.