Comparative Diagnostic Performance of Copeptin After Hypertonic Saline Infusion Versus Water Deprivation Test in Pediatric Patients with Polyuria-Polydipsia Syndrome.

Differentiating central diabetes insipidus (CDI), nephrogenic diabetes insipidus (NDI), and primary polydipsia (PP) in pediatric patients with polyuria-polydipsia syndrome (PPS) remains a clinical challenge. The water deprivation test (WDT) is the traditional gold standard; however, it is time-consuming, burdensome, and prone to equivocal results. Stimulated copeptin, a surrogate marker of vasopressin, has emerged as a promising diagnostic alternative. We conducted a prospective, observational, cross-sectional study involving 27 pediatric patients (ages 2-17) presenting with PPS. Each patient underwent a WDT with desmopressin and hypertonic saline infusion (3% NaCl) for stimulated copeptin testing. Diagnostic accuracy was assessed using clinical diagnoses as a reference. The WDT showed high accuracy with an area under the curve (AUC) of 0.97, and there was an increased optimal threshold of ≥14% urine osmolality after desmopressin acetate (1-deamino-8-D-arginine vasopressin, DDAVP) administration (sensitivity 88.9%, specificity 100%). Stimulated copeptin at a threshold of <6.5 pmol/L demonstrated 100% sensitivity and specificity (AUC = 1.00) for CDI versus PP. Basal copeptin ≥21.4 pmol/L accurately identified all NDI cases. The agreement between the WDT and copeptin was low (κ = 0.06, McNemar p = 0.021), suggesting that copeptin has greater specificity, particularly for borderline or partial CDI. These results support the use of stimulated copeptin as a first-line diagnostic tool in pediatric PPS, offering improved objectivity, tolerability, and diagnostic clarity compared with the WDT. Basal copeptin also demonstrated excellent performance in rapid noninvasive NDI identification.