Comparative study of two tests of renal diluting ability in Bartter's syndrome

1. Different results concerning distal NaCl reabsorption have been reported for patients with Bartter's syndrome in tests of renal diluting ability. We describe clearance studies performed on 3 patients with Bartter's syndrome using different routes for body fluid content expansion: water was given orally and 0.45% NaCl solution intravenously. The impact of fluid composition was evaluated in one patient who additionally underwent a "reverse test": i.e., intravenous 5% glucose in water and an oral load of 0.45% NaCl solution. 2. Urine flow per ml glomerular filtration rate (GFR) reached higher levels when the iv route was used (20.6 +/- 1.8 vs 11.8 +/- 5.7%, P < 0.05). Fractional excretion of Na+, Cl- and osmoles increased during NaCl infusion but not during the oral load. Also, distal delivery of solute increased and was greater than that observed in the oral test (21.9 +/- 5.5 vs 11.4 +/- 2.1%, P < 0.05). 3. In contrast, fractional distal chloride reabsorption in the iv test reached subnormal values which were lower than in the oral load test (65.0 +/- 11.2 vs 86.8 +/- 11.0%, P < 0.05). A positive correlation was observed between distal delivery and Cl- fractional excretion (r = 0.87; P < 0.005). In one patient, the 5% glucose infusion resulted in greater urine flow and distal delivery when compared to distilled water or 0.45% NaCl taken orally (28.1 vs 13.3 ml/min and 27.3 vs 12.8%, respectively). These values were as high as those observed during iv administration of hypotonic saline. 4. The iv route was always associated with lower rates of fractional distal chloride reabsorption (70.7 vs 89.1%) regardless of the solute composition and should be recommended when testing the renal diluting ability of patients suspected of Bartter's syndrome.