In this case, the copeptin level measured after hypertonic saline infusion proved to be more accurately and more reliable to differentiate primary polydipsia from central diabetes insipidus (Fenske et al.—Copeptin cutoff > 4.9 pmol/L: sensitivity = 93.2% [95% CI: 83.5–98.1], specificity = 100% [95% CI: 95.5–100.0], AUC = 0.97 [95% CI: 0.93–1.00]) than the water-deprivation test with or without copeptin measurement. This evidence concerns the gene AVP and Central diabetes insipidus.