Differential diagnosis of polyuric/polydipsic syndromes with the aid of urinary vasopressin measurement in adults

OBJECTIVE A water deprivation test or a hypertonic saline infusion test wit h the measurement of plasma osmolality and plasma vasopressin are the gold standard tests in the differential diagnosis of polyuric syndromes. Because commercially available vasopressin kits are too insensitive for this appro ach, and the concentration of vasopressin in urine is much higher than in p lasma, urinary vasopressin measurements may be an alternative to the more d ifficult plasma vasopressin measurement. DESIGN The diagnostic value of the measurement of urinary vasopressin with a rather insensitive commercially available vasopressin kit was compared wi th plasma vasopressin measurement by a highly sensitive radioimmunoassay (R IA). PATIENTS AND METHODS Thirteen normal subjects and 27 patients with polyuria /polydipsia were examined by an 8-h fluid deprivation test. In all blood sa mples (0800 h, 1200 h, 1400 h and 1600 h) and in all urine collections (2-h ourly fractions), osmolality as well as vasopressin were measured. RESULTS Using plasma vasopressin measurement with a highly sensitive RIA as gold standard test, nine patients were classified as having primary polydi psia, whereas 18 had partial or complete cranial diabetes insipidus. Wherea s the substitution of plasma vasopressin measurement by urinary vasopressin measurement alone did not provide 100% separation between both groups, the product of urinary vasopressin and urinary osmolality related to plasma os molality completely separated the patients with primary polydipsia from tho se with diabetes insipidus. Urinary measurement of vasopressin and osmolali ty alone, which was recommended as a noninvasive diagnostic procedure in ch ildren, was too insensitive for exact differential diagnosis in our adult p atients. CONCLUSIONS The simultaneous measurement of plasma vasopressin and plasma o smolality in a dehydration test is the most powerful diagnostic tool in the differential diagnosis of polyuria/polydipsia. However, if highly sensitiv e assays for plasma vasopressin measurements are not available, the measure ment of urinary vasopressin with commercially available, less sensitive RIA s may be a diagnostic alternative, which showed nearly the same sensitivity as plasma vasopressin measurement in our study population.