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Background: Chronic kidney disease with metabolic acidosis is common in older people, but the effectiveness of oral sodium bicarbonate therapy in this group is unclear. We tested whether oral sodium bicarbonate provides net health benefit for older people with advanced chronic kidney disease and serum bicarbonate concentrations < 22 mmol/L.Methods: Pragmatic multicentre, parallel group, double-blind, placebo-controlled randomised trial. We recruited adults aged ≥ 60 years with estimated glomerular filtration rate of < 30 mL/min/1.73 m2, not receiving dialysis, with serum bicarbonate concentration < 22 mmol/L, from 27 nephrology and geriatric medicine departments in the UK. Participants received oral sodium bicarbonate (up to 3 g/day) or matching placebo given for up to 2 years, randomised in a 1:1 ratio. The primary outcome was between-group difference in the Short Physical Performance Battery (SPPB) at 12 months, adjusted for baseline values, analysed by intention to treat. Secondary outcomes included generic and disease-specific quality of life (EQ-5D and KDQoL tools), anthropometry, renal function, walk distance, blood pressure, bone and vascular health markers, and incremental cost per quality-adjusted life year gained.Results: We randomised 300 participants between May 2013 and February 2017, mean age 74 years, 86 (29%) female. At 12 months, 116/152 (76%) participants allocated to bicarbonate and 104/148 (70%) allocated to placebo were assessed; primary outcome data were available for 187 participants. We found no significant treatment effect for the SPPB: bicarbonate arm 8.3 (SD 2.5) points, placebo arm 8.8 (SD 2.2) and adjusted treatment effect - 0.4 (95% CI - 0.9 to 0.1, p = 0.15). We found no significant treatment effect for glomerular filtration rate (0.6 mL/min/1.73 m2, 95% CI - 0.8 to 2.0, p = 0.39). The bicarbonate arm showed higher costs and lower quality of life as measured by the EQ-5D-3L tool over 1 year (£564 [95% CI £88 to £1154]); placebo dominated bicarbonate under all sensitivity analyses. Adverse events were more frequent in those randomised to bicarbonate (457 versus 400).Conclusions: Oral sodium bicarbonate did not improve physical function or renal function, increased adverse events and is unlikely to be cost-effective for use by the UK NHS for this patient group.Trial Registration: European Clinical Trials Database (2011-005271-16) and ISRCTN09486651; registered 17 February 2012. [ABSTRACT FROM AUTHOR]