Prevalence and characterization of renal tubular acidosis in patients with osteopenia and osteoporosis and in non-porotic controls

Background. Chronic metabolic acidosis may increase alkali mobilization from the bone and thus promote the development of osteoporosis. The objective of the current study was to compare urinary acidification in patients with reduced bone mineral content with that in control subjects with normal bone density. Methods. Forty-six subjects (41 females, 5 males) with osteopenia or osteoporosis were studied. In none of the subjects were overt metabolic acidosis, derangement of potassium homeostasis, or renal insufficiency present. Distal tubular acidification was studied by means of oral ammonium chloride loading test (0.1 g/kg body weight) and the oral frusemide test (40 mg). In addition the frusemide test was performed in 20 healthy age- and sex-matched controls (17 females, 3 males). Results. In all control subjects a urinary pH <5.5 was observed following the ingestion of 40 mg frusemide. In contrast, in patients with reduced bone mineral density incomplete renal tubular acidosis type I (RTA I) was diagnosed in 10 of 46 subjects (22%) by oral ammonium chloride loading lest. Disorders possibly related to RTA. I were detected in eight of these 10 patients. Thirty-six patients had a normal urinary pH response following oral ammonium chloride loading. Oral frusemide, 40 mg, failed to lower urinary pH <5.5 in sixteen patients (35%), these included 10 subjects with incomplete RTA I, and six subjects with a normal oral ammonium chloride loading test. An abnormal frusemide test was found in 35% of patients with reduced bone mass and in none of the normal controls (chi(2)=7.39; P<0.01). With the ammonium chloride test as the gold standard for diagnosis of distal RTA, the frusemide test showed a sensitivity of 1.0 (95% CI, 0.69-1.0) and a specificity of 0.89 (95% CI, 0.78-0.96) for the diagnosis of distal RTA. Patients with incomplete RTA I were younger than those without incomplete RTA I (42+/-16 vs 54+/-14 years; P=0.025; mean+/-SD). Basal serum bicarbonate concentrations and capillary pH did not differ between the groups. Conclusion. Incomplete RTA I may be prevalent in a significant proportion of patients suffering from osteopenia or osteoporosis. The outcome of the frusemide test suggests either a defect of the H+ ATPase in the cortical collecting tubule (CCT) or a defective Na+ reabsorption in the CCT. Prospective studies are needed to further elucidate the impact of incomplete RTA I on the development of reduced bone mineral content.