Longitudinal serum bicarbonate and mortality risk in older patients with advanced chronic kidney disease: analyses from the EQUAL cohort

Background
We aimed to explore the relationship between serum bicarbonate (SBC) and mortality in advanced chronic kidney disease (CKD) during three distinct treatment periods: during the pre-kidney replacement therapy (KRT) period, during the transition phase surrounding the start of KRT (transition-CKD) and during KRT.

Methods
Using the European QUALity Study on treatment in advanced CKD (EQUAL) cohort, which includes patients aged ≥65 years and estimated glomerular filtration rate (eGFR) ≤20 mL/min/1.73 m2 from six European countries, we explored the association between longitudinal SBC and all-cause mortality in three separate CKD populations: pre-KRT, transition-CKD and in the KRT populations, using multivariable time-dependent Cox regression models. We evaluated effect modification by pre-specified variables on the relationship between SBC and mortality.

Results
We included 1485 patients with a median follow-up of 2.9 (interquartile range 2.7) years, during which 529 (35.6%) patients died. A U-shaped relationship between SBC levels and all-cause mortality was observed in the pre-KRT population (P = .03). Low cumulative exposure, defined as the area under the SBC trajectory before KRT initiation, was associated with increased mortality risk after transitioning to KRT (P = .01). Similarly, in the KRT population, low SBC levels showed a trend towards increased mortality risk (P = .13). We observed effect modification by subjective global assessment category (P-value for interaction = .02) and KRT (P-value for interaction = .02).

Conclusions
A U-shaped relationship describes the association between SBC and mortality in the advanced CKD pre-KRT population, whereas in the KRT population a trend towards an increased mortality risk was observed for low SBC levels.