A Reliable Diagnostic Test for Hypotonic Polyuria

Pinpointing the cause of hypotonic polyuria in a patient poses a diagnostic and therapeutic challenge, particularly in distinguishing primary polydipsia (either psychogenic or nonpsychogenic) from partial pituitary insufficiency due to injury or neoplasm. Diabetes insipidus is one cause of hypotonic polyuria and previously was classified as either central or nephrogenic, on the basis of a water-deprivation test and the administration of vasopressin.1 But in 1970, Miller et al. noted that some patients who underwent water deprivation had a urine osmolality greater than their plasma osmolality and that the urine osmolality further increased, albeit modestly, after the injection of vasopressin — . . .