Copeptin in the Diagnosis and Differential Diagnosis of Diabetes Insipidus. The CoSIP-Study

Background:

Plasma arginine vasopressin (AVP) measurement is recommended for the differential diagnosis of diabetes insipidus and polydipsia. However, AVP measurement is cumbersome. AVP is derived from a larger precursor peptide along with copeptin, which is a more stable peptide directly mirroring the production of AVP. Copeptin can be assayed readily in plasma.

Aim: To evaluate the diagnostic accuracy of copeptin levels in the diagnosis and differential diagnosis of diabetes insipidus.

Design: Prospective, observational multicenter study.

Setting: Department of Endocrinology, University Hospital of Basel

Patients: Patients with suspected or known central (complete or partial), nephrogenic (complete or partial) or psychogenic diabetes insipidus undergoing a standardized water deprivation test.

Intervention: All patients with suspected or known diabetes insipidus will undergo an overnight water deprivation test and a standardized water deprivation test, as routinely performed in the diagnostic evaluation of diabetes insipidus. Plasma AVP and copeptin will be measured at baseline (8 am before start of thirsting), and hourly during the water deprivation test.

Study hypothesis: Copeptin levels will provide a better diagnostic accuracy in the diagnosis and differential diagnosis of diabetes insipidus as compared to AVP measurement.

Analysis: We will study 5 groups of patients: A) Patients with complete central diabetes insipidus, B) Patients with partial central diabetes insipidus, C) Patients with complete nephrogenic diabetes insipidus, D) Patients with partial nephrogenic diabetes insipidus and E) Patients with psychogenic diabetes insipidus. All groups will consist of 10 patients based on the following assumptions: Based on pilot studies we assume that patients in group A) will have copeptin values of 2.5 ± 1.0; Group B) 3.0 ± 1.0, Group C) 15.0 ± 5; Group D) 6 ± 2.0 and Group E) 4.0 ± 1.0 pmol/L. This results in a power of 90% to detect a difference in copeptin levels of 0.8pmol/L between the closest two groups, i.e. patients with partial central Diabetes insipidus and patients with psychogenic Diabetes insipidus.

Significance: The measurement of copeptin will allow a better discrimination of patients with diabetes insipidus, especially for the discrimination of partial central and nephrogenic and psychogenic diabetes insipidus.

Inclusion criteria

• All patients > 18 years who are tested for the diagnosis of DI because of a history of polyuria (> 40 ml/kg per 24 hours) in the presence of polydipsia Patients with known DI will be contacted whether they agree to participate in the study and to undergo again a water deprivation test to measure copeptin to confirm the diagnosis.

Exclusion criteria

• Polyuria of other origin, i.e. prostate hyperplasia, diabetes mellitus.
• Pregnancy
• The investigators do not perform the water deprivation test in patients with: *renal insufficiency

• uncontrolled diabetes mellitus
• hypovolemia of any cause
• uncorrected deficiency of adrenal or thyroid hormones
• No informed consent