Abstract 40

An abnormal urinary/plasma osmolality ratio (UOsm/POsm) less than 1 after a water deprivation test that reveals an incapacity for water retention, is a strong criteria for the diagnosis of DI. However, this ratio does not discriminate well between patients with a partial DI from those with a severe Primary Polydipsia (PP). Objective: To determine the usefulness of measuring plasma ADH concentration in children and adolescents with polyuria and polydipsia as a tool to differentiate CDI from those with PP. Twenty patients between 0.2 and 17.7 years of age with polyuria and polydipsia were studied. Fifteen patients underwent a water deprivation test. The other five were studied only on basal condition since their plasma were already hyperosmolar. Urine and plasma osmolarity were measured by freezing point determination (Fiske Osmometer). We considered an UOsm/POsm ratio less than 1 as abnormal, characteristic of DI, a normal ratio if greater than 2, while between 1 and 2 was considered a partial incapacity to retain water. Antidiuretic hormone concentration was measured by RIA (DSL) in methanol extracted plasma using silice column (C18). The relationship between ADH and POsm was plotted in a nomogram developed with data of our normal population. Results: Twelve patients had a UOsm/POsm ratio less than 1 (X±SD:0.41± 0.17). In ten of them ADH was abnormally low 1.4±9 pg/ml confirming the diagnosis of complete CDI. One patient with a low ratio (0.55) and a normal ADH (5.4 pg/ml) had a PP. Another patient with a ratio of 0.30 and a plasma ADH of 3.1 pg/ml had a partial CDI. In 2 patients with a ratio greater than 2, ADH fell within the normal range (7.6 and 10.1 pg/ml) confirming the diagnosis of PP. In six patients the UOsm/POsm ratios were within 1 and 2. One of them had a low ADH (1.6 pg/ml) confirming a partial CDI, while in the other five their plasma ADH levels fell within the normal range according to their POsm suggesting the diagnosis of PP. Conclusions: The measurement of plasma ADH confirms the diagnosis of CDI, but it became a better aid in the identification of patients with partial CDI or severe PP, where the latter show a reversible incapacity to concentrate urine.