Trials for Simplified Hypertonic Saline Test

Hypertonic saline test is indispensable for the evaluation of posterior pituitary function. However the test is not simple, including water loading, urine sampling and at least 45 min of hypertonic saline infusion, mostly because the test relies on urinary osmolality as an index of ADH secretion. The object of this study is try to simplify the test by directly measuring plasma ADH concentration before and after 10 min of hypertonic saline infusion. Intravenous infusion of hypertonic saline (5% NaCl, 0.24 ml/kg/min, for 10 min) was performed on normal subjects, patients with diabetes insipidus and patients with renal failure under chronic hemodialysis. Venous blood samples were obtained seriously including just before and after 10 min of the infusion. ADH was extracted from plasma using Sep-Pak C18 column and assayed by specific RIA. Minimum sensitivity of the assay was 0.25 pg/ml.The hypertonic saline infusion resulted in an increase of plasma osmolality by about 8 mOsm/kg H2 0 and plasma sodium concentration by 4 mEq/l. Plasma ADH increased from 0.77 ± 0.09 to 3.42 ± 0.73 pg/ml (m ± SE, n = 8, p<0.01) in normal subjects of ad lib. water drinking and from 0.55 ± 0.33 to 2.34 ± 0.33 (m ± SE, n = 4, p<0.05) in water loaded normal subjects (20 ml/kg of water, 60 min before hypertonic saline infusion). In patients with diabetes insipidus, plasma ADH level was low (0.63 ± 0.24 pg/ml, n = 5) relative to plasma osmolality (295.4 ± 4.6 mOsm/kg H2 0) and showed no significant increase after hypertonic saline infusion. In patients with renal failure, plasma ADH increased from 2.47 ± 0.28 to 4.14 ± 1.09 pg/ml. Thus basal ADH level was significantly higher than normal subjects but response to hypertonic saline was reduced.These results suggested that this simplified test with 10 min infusion of hypertonic saline, no water loading and no urine sampling may be useful for the evaluation of ADH secretion and differential diagnosis of polyuria.