Copeptin Level Distinguishes Diabetes Insipidus from Primary Polydipsia

Distinguishing between diabetes insipidus and primary polydipsia sometimes is difficult; both disorders present with polyuria and dilute urine. Two diagnostic approaches are the water-deprivation test (with serial measurements of urine and plasma osmolality) and the response to administration of hypertonic saline, but both approaches have limitations. Direct measurement of the response of plasma antidiuretic hormone (ADH, or arginine vasopressin) to provocative testing is potentially helpful, but assays for ADH can be unreliable. Copeptin, a fragment that is cleaved from the ADH prohormone, is more stable and easier to measure.

In this study, researchers studied 141 patients with hypotonic polyuria; ultimately, 58% received diagnoses of prim…