Serum Bicarbonate as a Surrogate for pH in Hemodialysis: A Pilot Study

Rationale & Objective: Excess morbidity and mortality are associated with both high and low serum bicarbonate levels in epidemiologic studies of patients with end-stage kidney disease (ESKD) receiving hemodialysis. The Kidney Disease Outcomes Quality Initiative (KDOQI) recommends modifying dialysate bicarbonate concentration to achieve a predialysis serum bicarbonate level ≥ 22 mmol/L, measured as total carbon dioxide (CO₂). This practice assumes that total CO₂ is an adequate surrogate for acid-base status, yet its surrogacy performance is unknown in ESKD. We determined acid-base status at the beginning and end of hemodialysis using total CO₂ and pH and tested whether total CO₂ is an appropriate surrogate for acid-base status. Study Design: Pilot study. Setting & Participants: 25 veterans with ESKD receiving outpatient hemodialysis. Tests Compared: pH, calculated bicarbonate level, and total CO₂. Outcomes: The proportion of paired samples for which total CO₂ misclassified acid-base status according to pH was determined. Bias of total CO₂ was evaluated using Bland-Altman plots, comparing it to calculated bicarbonate. Results: Among 71 samples, mean pH was 7.41 ± 0.03 predialysis and 7.48 ± 0.05 postdialysis. Compared with interpretation of full blood gas profiles, 9 of 25 (36%) participants were misclassified as acidemic using predialysis total CO₂ measures alone (total CO₂ < 22 mmol/L but pH ≥ 7.38); 1 (4%) participant was misclassified as alkalemic (total CO₂ > 26 mmol/L but pH ≤ 7.42). Among paired samples in which predialysis total CO₂ was < 22 mmol/L, the corresponding pH was acidemic (< 7.38) in just 3 of 13 (23%) instances. Limitations: Small, single-center, entirely male cohort. Conclusions: A majority of participants became alkalemic during routine hemodialysis despite arriving with normal pH. 10 of 25 (40%) participants’ acid-base status was misclassified using total CO₂ measurements alone; the majority of predialysis total CO₂ values that would trigger therapeutic modification according to practice guidelines did not have acidemia when assessed using pH. Efforts to improve dialysis prescription require recognition that total CO₂ may not be reliable for interpreting acid-base status in hemodialysis patients.