AVP and Hyponatremia: In 2009 Fenske et al. demonstrated through a prospective study a very high variability in copeptin levels of 106 hyponatremic patients, but suggested that the ratio between plasma copeptin and urinary sodium (P(copeptin)-to-U(Na+) ratio), could help distinguishing SIADH-related hyponatremia from that following either real or virtual volume depletion (hypovolemic and hypervolemic, respectively) [14].