(HealthDay News) — In patients with hypotonic polyuria, the direct measurement of hypertonic saline-stimulated plasma copeptin has better diagnostic accuracy than the water-deprivation test, according to a study published in the New England Journal of Medicine.

Wiebke Fenske, MD, PhD, from the University of Leipzig in Germany, and colleagues compared the indirect water-deprivation test with direct detection of plasma copeptin among 144 patients with hypotonic polyuria at 11 medical centers who underwent both tests.

The researchers found that the indirect water-deprivation test determined the correct diagnosis in 108 of the 141 patients included in the analysis (diagnostic accuracy, 76.6%), while the hypertonic saline infusion test (with a copeptin cutoff level of >4.9 pmol per liter) determined the correct diagnosis in 136 patients (96.5%). Primary polydipsia was distinguished from partial central diabetes insipidus with the indirect water-deprivation test in 77 of 105 patients (73.3%), compared to 99 of 104 patients (95.2%) with the hypertonic saline infusion. One serious adverse event (desmopressin-induced hyponatremia), which required hospitalization, occurred during the water-deprivation test.

“This prospective evaluation of patients with hypotonic polyuria validated hypertonic saline-stimulated copeptin measurement as a diagnostic method that appeared to be superior to the indirect water-deprivation test in distinguishing central diabetes insipidus from primary polydipsia,” the authors write.

Several authors disclosed financial ties to pharmaceutical and medical device companies, including Thermo Fisher Scientific, which funded laboratory measurement of copeptin.

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Fenske W, Refardt J, Chifu I, et al.A Copeptin-Based Approach in the Diagnosis of Diabetes Insipidus. N Engl J Med 2018;379:428-439DOI: 10.1056/NEJMoa1803760

Rosen CJ, and Ingelfinger JR. A Reliable Diagnostic Test for Hypotonic Polyuria. N Engl J Med 2018;379:483-484. DOI: 10.1056/NEJMe1808195