[New Diagnostic Standard in Diabetes Insipidus]

2018 brings new insights into the differential diagnosis of patients with hypotonic polyuria syndrome. Exact localization of the antidiuretic defect in patients with hypotonic polyuria can be challenging, especially the distinction of primary polydipsia from partial pituitary or renal forms of diabetes insipidus. However, diagnostic precision is vital since therapeutic management differs substantially and false treatment may result in serious consequences.A main limitation of the indirect water deprivation test is its limited accuracy to correctly distinguish between the pathophysiologically distinct forms of hypotonic polyuria with a residual antidiuretic capacity. Direct detection of the osmotically-sensitive AVP reserve may be diagnostically superior but is technically difficult and not available for clinical routine, having left indirect diagnostic readouts the only accepted method for differentiating polyuric states, despite a diagnostic accuracy of only 70 %.New data demonstrate that selective measurement of plasma copeptin, a reliable AVP surrogate, not only captures patients with underlying renal defect, but critically improves diagnostic differentiation between primary polydipsia and pituitary forms of central diabetes insipidus.This manuscript presents the novel findings in the field and interprets their clinical consequences.