AVP and Down syndrome: Dosing AVP is of limited usefulness, as the levels of plasma of AVP have been proven to fluctuate unpredictably and are often augmented in patients with both true hypovolemia (decreased ECFV) and effective hypovolemia (increased ECFV), as a result of a non-osmotic stimulus, as well as in the syndrome of inappropriate antidiuretic hormone secretion (SIADH), which remains in essence, a diagnosis of exclusion.