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

Abstract

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.

Defining Acid-Base Status in Hemodialysis: Is Bicarbonate Enough?

“…Avoidance of large rapid perturbations in acid-base status appears prudent, but proposals to avoid such perturbations by using lower bicarbonate concentrations in the dialysate must address the potential long-term risks of predialysis metabolic acidosis. This report by Woodell et al 6 provides further evidence of the limitations of relying solely on the predialysis serum bicarbonate level. Future studies should include the following objectives: (1) continue to define the optimal method to assess acid-base status in hemodialysis patients, perhaps using a patient-centered rather than a one-size-fits-all approach; and (2) determine whether more accurate assessment improves clinical decision making and whether this translates into improved clinical and patient-centered outcomes.…”

“…In this issue of Kidney Medicine, Woodell et al 6 prospectively evaluated the utility of serum bicarbonate as the sole measurement determining acid-base treatment decisions. They enrolled 25 male hemodialysis patients and assessed acid-base status pre-and postdialysis for 3 months, using both blood gas and chemistry measurements.…”

Total Carbon Dioxide Versus pH for Determining Acid-Base Status in Patients on Continuous Kidney Replacement Therapy: A Cohort Study

“…2 We recently showed in patients receiving maintenance hemodialysis that compared to pH, 15% of inferences about acid-base status using tCO 2 measures were incorrect. 3 Notably, tCO 2 measurements were systematically lower than blood gas-calculated bicarbonate (HCO 3 − ). Whether this difference represents local laboratory patterns or more universal observations is unknown.…”

“…Paired samples were defined as misclassified if tCO2 suggested an acid-base status other than that measured by pH (eg, tCO 2 low [suggesting acidosis] but pH normal or alkalotic). The difference between tCO 2 and calculated HCO 3 − was compared between paired samples with and without misclassified acidosis using linear regression with robust variance estimation, adjusting for PCO 2 and patient factors. Analyses were performed using STATA 15 (StataCorp).…”

“…This difference is reported in the general population 4 and more recently in patients receiving maintenance hemodialysis. 3 These independent observations across distinct patient populations collectively question theoretical assumptions about equivalence of tCO 2 and calculated HCO 3 − in clinical settings. Although misclassification may result from chronic respiratory or mixed acid-base disorders, our results suggest misclassification may also occur from sample handling issues causing spurious results for tCO 2 or HCO 3…”