Copeptin is not useful as a marker of malignant disease in the syndrome of inappropriate antidiuresis

OBJECTIVE: The syndrome of inappropriate antidiuresis (SIAD) is a common condition in hospitalized patients. It is crucial to establish the cause of SIAD, especially in order to exclude underlying malignancy. As malignant SIAD may be due to a paraneoplastic synthesis of arginine vasopressin, we hypothesized that its stable surrogate marker copeptin can be used as a diagnostic tool to differentiate between malignant and non-malignant SIAD. METHODS: Prospective observational study. We analyzed data from 146 SIAD patients of two different cohorts from Switzerland and Germany. Patients were included while presenting at the emergency department and underwent a standardized diagnostic assessment including the measurement of copeptin levels. RESULTS: Thirty-nine patients (median age: 63 years, 51% female) were diagnosed with cancer-related SIAD and 107 (median age: 73 years, 68% female) with non-malignant SIAD. Serum sodium levels were higher in cancer-related versus non-malignant SIAD: median (IQR) 124 mmol/l (120; 127) versus 120 mmol/l (117; 123) (P<0.001). Median (IQR) copeptin levels of patients with cancer-related SIAD were 11.1 pmol/l (5.2; 37.1) and 10.5 pmol/l (5.2; 25.2) with non-malignant SIAD (P = 0.38). Among different cancer entities, patients suffering from small-cell lung cancer showed the highest copeptin values, but overall no significant difference in copeptin levels between cancer types was observed (P = 0.46). CONCLUSIONS: Copeptin levels are similar in cancer-related and non-malignant SIAD. Therefore, Copeptin does not seem to be suitable as a marker of malignant disease in SIAD.