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Background: Copeptin is used in the differential diagnosis of diabetes insipidus. Different copeptin immunoassays exist but inter-assay comparability is unclear. The aim of this study was to correlate three commercially available copeptin assays and their diagnostic accuracy in the differential diagnosis of the polyuria-polydipsia-syndrome. Methods: Analyzed data include three different studies: repeated copeptin measures of 8 healthy volunteers undergoing osmotic stimulation; copeptin measures of 40 patients hospitalized with pneumonia; osmotically stimulated copeptin measures of 40 patients with polyuria-polydipsia-syndrome. Copeptin was measured using the automated B.R.A.H.M.S. KRYPTOR, the manual B.R.A.H.M.S. LIA and the manual Cloud Clone ELISA assay. Primary outcome was the diagnostic accuracy in the polyuria-polydipsia-syndrome. Results: In total, 150 copeptin measurements were analyzed. In healthy volunteers, there was a moderate correlation for the KRYPTOR and LIA (interrater correlation coefficient (ICC) 0.74; 95%-CI 0.07-0.91), and a poor correlation for the KRYPTOR and ELISA (ICC 0.07; 95%-CI -0.06-0.29), as for the LIA and ELISA (ICC 0.04; 95%-CI -0.04-0.17). The KRYPTOR had the highest diagnostic accuracy (98% (95%-CI: 83-100)), comparable to the LIA (88% (95%-CI: 74-100)), while the ELISA had a poor diagnostic accuracy (55% (95%-CI: 34-68)) in the differential diagnosis of the polyuria-polydipsia-syndrome. Conclusion: The KRYPTOR and LIA yield comparable copeptin levels and a high diagnostic accuracy, while the ELISA correlates poorly with the other two assays and shows a poor diagnostic accuracy between polyuria-polydipsia patients. Redefining cut-off levels for copeptin assays other than KRYPTOR and LIA must take place before their use in the differential diagnosis of diabetes insipidus.

Validity of Different Copeptin Assays in the Differential Diagnosis of the Polyuria-Polydipsia Syndrome