Hypernatraemia (serum sodium > 145 mmol/L)

• No tests in the community: hospital admission for rehydration and monitoring of serum sodium.
• Serum sodium > 155 mmol/L, confusion, drowsiness, seizures, or inability to drink (swallowing disorders, cognitive impairment, vomiting).

Laboratory tests

• Serum electrolytes (repeat serum sodium, serum potassium)
• Plasma glucose
• Serum creatinine with eGFR
• Serum calcium
• FBC (if looking for infection or haemoconcentration)if needednon-French guidelineA review on hospitalised older patients puts the FBC in the work-up, to look for infection or haemoconcentration ^[2]. A Korean review detailing the diagnosis in eight steps does not cite it ^[1], and an American review has tests guided by the history when the cause is unclear ^[4].
• CRP (if looking for infection)if needednon-French guidelineA review on hospitalised older patients puts inflammatory markers in the work-up and cites acute infection among the causes of dehydration ^[2]. A Korean review detailing the diagnosis in eight steps does not cite them ^[1], and an American review has tests guided by the history when the cause is unclear ^[4].
• CK (if fall with prolonged time on the floor, or seizure)if needednon-French guidelineFall with prolonged time on the floor, seizure or muscle trauma: rhabdomyolysis can raise serum sodium by water shift into cells ^[2].
• Serum lithium (if on lithium)if needednon-French guidelinePatient treated with lithium: lithium causes nephrogenic and central diabetes insipidus ^[1,2].
• Spot urine osmolality (if no obvious cause or if polyuria)if needednon-French guidelineHypernatraemia without obvious losses or lack of intake, or abundant clear urine despite dehydration: distinguishes diabetes insipidus (< 300 mOsm/kg with polyuria) from extrarenal losses or lack of thirst (> 800 mOsm/kg) ^[1].

In the practice

• Urine dipstick (for glycosuria and urine specific gravity)if needednon-French guidelineA review on hospitalised older patients regards the dipstick as a useful bedside measurement, to assess urine concentration ^[2]. A Korean review detailing the diagnosis in eight steps asks for urine volume and osmolality, without citing the dipstick ^[1].