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 (CO(2)). This practice assumes that total CO(2) 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 CO(2) and pH and tested whether total CO(2) 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 CO(2). OUTCOMES: The proportion of paired samples for which total CO(2) misclassified acid-base status according to pH was determined. Bias of total CO(2) 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 CO(2) measures alone (total CO(2) < 22 mmol/L but pH ≥ 7.38); 1 (4%) participant was misclassified as alkalemic (total CO(2) > 26 mmol/L but pH ≤ 7.42). Among paired samples in which predialysis total CO(2) 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 CO(2) measurements alone; the majority of predialysis total CO(2) 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 CO(2) may not be reliable for interpreting acid-base status in hemodialysis patients.