Tolvaptan, an arginine vasopressin V2 antagonist, is available for patients with refractory ascites. Free water clearance was evaluated as a predictor of tolvaptan efficacy. Twenty‑one patients with refractory ascites were enrolled in the present study. Liver function test, renal function test, urine volume, free water clearance and osmotic pressure were measured at baseline (day 0) and for each dose of tolvaptan (1.875, 3.75 and 7.5 mg), and compared for efficacy. Tolvaptan increased urine volume and free water clearance decreased osmotic pressure at each dose of tolvaptan, compared to pretreatment levels. Compared to baseline, an increased volume of free water clearance at 1.875 mg of tolvaptan showed a significant correlation with body weight reduction (r=0.480 and P=0.028). Any factors (age, liver function test and renal function test) at pretreatment showed no significant correlation with body weight reduction. An increased volume of urine and osmotic pressure at each dose was not significantly correlated with the tolvaptan effect. Compared to baseline, an increased volume of free water clearance at 1.875 mg of tolvaptan in responders was significantly increased, compared to non‑responders (270±241 ml/day: 27±257 ml/day; P=0.042). In conclusion, an increased volume of free water clearance on day 1 was significantly associated with body weight reduction. Free water clearance could be a simple and useful marker for the prediction of tolvaptan efficacy.

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