Diabetes insipidus is characterized by hypoosmotic polyuria related to deficiency of arginine–vasopressin (AVP) secretion (central diabetes insipidus, CDI) or renal insensitivity to AVP (nephrogenic diabetes insipidus, NDI). The water deprivation test with assessment of AVP activity is currently the gold standard for differential diagnosis in patients presenting polyuria–polydipsia syndrome. Nevertheless, it can be dangerous without proper surveillance and its interpretation may be challenging. Other markers have been suggested. Direct quantification of circulating AVP is not sufficient for diagnosis: vasopressin is unstable, analysis is complex. AVP comes from prohormone preprovasopressin with concomitant release of copeptin (C-terminal moiety) in the equimolar ratio. Copeptin is stable in vitro, with easy and rapid measurement (<4h). Past studies have shown greater sensitivity and specificity of copeptin versus AVP to discriminate etiologies of polyuria in adults, but its value has not been demonstrated in infants yet.

A 7-month-old infant presented polyuria–polydipsia syndrome with poor weight gain. Laboratory tests pointed out hypernatremia (170mmol/L) and blood hyperosmolarity (330mOsm/L) with inappropriate urinary hypoosmolarity (168mOsm/L). Plasmatic copeptin measurement was found at a very high level, 303pmol/L (1–14pmol/L). DdAVP administration did not improve the polyuria, confirming the final diagnosis of NDI. Hyperhydration with a hypoosmolar diet normalized the hydration status and circulating levels of copeptin within 1 week.

Copeptin, a stable peptide reflecting AVP secretion, could be a safer and faster biomarker for etiological diagnosis of polyuria–polydipsia syndrome in children. Before regularization of hydration status, a single baseline measurement may be enough to discriminate NDI from other etiologies without the water deprivation test.