How can I differentiate between hypernatraemia due to dehydration versus other causes such as diabetes insipidus?

To differentiate hypernatraemia caused by dehydration from hypernatraemia due to other causes such as diabetes insipidus, assess the patient's volume status, urine osmolality, and urine sodium concentration.

• Dehydration-related hypernatraemia typically presents with signs of hypovolaemia such as dry mucous membranes, reduced skin turgor, tachycardia, and decreased urine output. Clinical features of dehydration and hypernatraemic dehydration include sunken eyes, lethargy, irritability, and in severe cases, convulsions or coma. Laboratory tests often show concentrated urine with high urine osmolality and low urine sodium due to renal sodium retention to conserve volume ³.
• Diabetes insipidus (DI) causes hypernatraemia due to free water loss from inability to concentrate urine. Patients usually have polyuria and polydipsia. Urine osmolality is inappropriately low (dilute urine) despite hypernatraemia, and urine sodium is typically low. Volume status may be normal or hypovolaemic depending on fluid intake ¹.
• Measuring urine osmolality and sodium helps distinguish between hypovolaemic hypernatraemia (dehydration) and euvolaemic or hypervolaemic hypernatraemia caused by DI or other causes. In dehydration, urine is concentrated; in DI, urine is dilute despite hypernatraemia ².
• Clinical assessment of volume status combined with laboratory investigations (serum sodium, urine osmolality, urine sodium) is essential for differentiation and guiding management ¹,²,³.