The direct measurement of hypertonic saline–stimulated plasma copeptin had greater diagnostic accuracy than the water-deprivation test in patients with hypotonic polyuria. A study published online August 2, 2018 in the New England Journal of Medicine showed that the indirect water-deprivation test correctly distinguished primary polydipsia from partial central diabetes insipidus in 77 of 105 patients and the hypertonic saline infusion test distinguished between the two conditions in 99 of 104 patients.