Serum bicarbonate and long-term outcomes in CKD

Background: A low serum bicarbonate level is prevalent in chronic kidney disease (CKD); however, its relationship to long-term outcomes is unclear. Study Design: Cohort study. Setting & Participants: The Modification of Diet in Renal Disease (MDRD) Study examined the effects of dietary protein restriction and blood pressure control on progression of kidney disease. This analysis includes 942 screened but nonrandomized individuals and 839 randomized participants with baseline serum bicarbonate measurements with stage 2-4 CKD. Factor: Serum bicarbonate level categorized into quartiles. Outcomes: Kidney failure, all-cause mortality, and a composite outcome of mortality and kidney failure. Measurements: Local laboratories at each participating site measured bicarbonate in fasting serum samples. Kidney failure outcomes were obtained from the US Renal Data System, and mortality data, from the National Death Index. Results: Mean glomerular filtration rate (GFR) was 39 ± 21 (SD) mL/min/1.73 m² and serum bicarbonate level was 23.3 ± 3.8 mEq/L. Kidney failure rates were 72%, 64%, 50%, and 41%; mortality rates were 31%, 25%, 21%, and 25%, and rates of the composite outcome were 78%, 71%, 58%, and 54% in bicarbonate quartiles 1, 2, 3, and 4, respectively. In analyses adjusted for demographic and cardiovascular disease factors, serum albumin level, proteinuria, and cause of kidney disease, compared with quartile 4, quartile 1 was associated with a 2.22 HR (95% CI, 1.83-2.68) of kidney failure; 1.39 HR (95% CI, 1.07-1.18) of all-cause mortality; and 1.36 HR (95% CI, 1.15-1.62) of the composite outcome. These associations were rendered nonsignificant with adjustment for GFR (kidney failure HR, 1.05 [95% CI, 0.87-1.28]; all-cause mortality HR, 0.99 [95% CI, 0.75-1.13]; composite HR, 1.04 [95% CI, 0.87-1.24]). Limitations: Single baseline measurement of serum bicarbonate. Conclusions: Low serum bicarbonate level was associated with increased risk of long-term outcomes in nondiabetic patients with CKD. However, this risk is not independent of baseline GFR. Clinical trials are necessary to evaluate whether bicarbonate supplementation slows the progression of CKD.