With the advent of vaptans as a therapeutic option, rapid and precise diagnosis of SIADH is essential. SIADH due to paraneoplastic stimulation of AVP is an expected but not the only cause of hyponatraemia in cancer patients. Therefore a precise evaluation of the cause of hyponatraemia is necessary before any therapeutic approach. Using Copeptin, a stable marker of AVP, which is stoichiometrically released with AVP, we evaluated 49 patients with hyponatremia due to various forms of cancer. We followed classical diagnostic criteria for the assement of hyponatraemia which included clinical evaluation of the fluid status, measurement of serum cortisol, thyroid hormones, glucose, serum and urine osmolality and urinary sodium excretion. Only patients with hyponatremia of <130 mmol/l were selected for the study. Serum copeptin levels were measured in addition to the above investigations with a specific immunoassay (detection limit of 1.7 pmol/l). The maximal inter and intra-assay CV being 6.5% for copeptin.
Compared to a control group and to the physiological variation found in the pattern of 24 h rhythm of copeptin obtained by sampling every 20 min in healthy subjects, 20 patients exceeded the maximal range of copeptin obtained in these healthy controls. The mean copeptin value of these patients (36 pmol/l) is more than twice the upper limit of normal of healthy subjects (14 pmol/l). Copeptin levels in the remaining group of patients was not significantly different to the control group.
Our data support the idea that measurement of serum copeptin levels may help to rapidly categorize SIADH as the cause of hyponatremia in cancer patients as compared to other causes of hyponatremia. These determinations may guide the decision for treatment with blockers of the AVP receptor.