Arginine or Hypertonic Saline–Stimulated Copeptin to Diagnose AVP Deficiency

BACKGROUND: Distinguishing between arginine vasopressin (AVP) deficiency and primary polydipsia is challenging. Hypertonic saline-stimulated copeptin has been used to diagnose AVP deficiency with high accuracy but requires close sodium monitoring. Arginine-stimulated copeptin has shown similar diagnostic accuracy but with a simpler test protocol. However, data are lacking from a head-to-head comparison between arginine-stimulated copeptin and hypertonic saline-stimulated copeptin in the diagnosis of AVP deficiency. METHODS: In this international, noninferiority trial, we assigned adult patients with polydipsia and hypotonic polyuria or a known diagnosis of AVP deficiency to undergo diagnostic evaluation with hypertonic-saline stimulation on one day and with arginine stimulation on another day. Two endocrinologists independently made the final diagnosis of AVP deficiency or primary polydipsia with use of clinical information, treatment response, and the hypertonic-saline test results. The primary outcome was the overall diagnostic accuracy according to prespecified copeptin cutoff values of 3.8 pmol per liter after 60 minutes for arginine and 4.9 pmol per liter once the sodium level was more than 149 mmol per liter for hypertonic saline. RESULTS: Of the 158 patients who underwent the two tests, 69 (44%) received the diagnosis of AVP deficiency and 89 (56%) received the diagnosis of primary polydipsia. The diagnostic accuracy was 74.4% (95% confidence interval CI, 67.0 to 80.6) for arginine-stimulated copeptin and 95.6% (95% CI, 91.1 to 97.8) for hypertonic saline-stimulated copeptin (estimated difference, -21.2 percentage points; 95% CI, -28.7 to -14.3). Adverse events were generally mild with the two tests. A total of 72% of the patients preferred testing with arginine as compared with hypertonic saline. Arginine-stimulated copeptin at a value of 3.0 pmol per liter or less led to a diagnosis of AVP deficiency with a specificity of 90.9% (95% CI, 81.7 to 95.7), whereas levels of more than 5.2 pmol per liter led to a diagnosis of primary polydipsia with a specificity of 91.4% (95% CI, 83.7 to 95.6). CONCLUSIONS: Among adult patients with polyuria polydipsia syndrome, AVP deficiency was more accurately diagnosed with hypertonic saline-stimulated copeptin than with arginine-stimulated copeptin. (Funded by the Swiss National Science Foundation; CARGOx ClinicalTrials.gov number, NCT03572166.).