AVP and Hyponatremia: While plasma CoP measurement has tremendous value in the differential diagnosis of PUPD, its diagnostic utility is limited in the case of hyponatremia where no differentiation between patients with a diagnosis of Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) and other etiologies leading to hyponatremia seems possible (2, 122), with the possible exception of cerebral salt-wasting syndrome (CSWS) (1).