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Abstract: TH-PO334

Misclassification of Acid-Base Status Using Serum Bicarbonate in Hemodialysis

Session Information

Category: Dialysis

  • 701 Dialysis: Hemodialysis and Frequent Dialysis


  • Woodell, Tyler, Oregon Health & Science University, Portland, Oregon, United States
  • Rifkin, Dena E., University of California, San Diego, La Jolla, California, United States
  • Ellison, David H., Oregon Health & Science University, Portland, Oregon, United States
  • Weiss, Jessica W., Oregon Health & Science University, Portland, Oregon, United States

Acid-base status is a potentially modifiable determinant of mortality among ESRD patients. Little is known about how acid-base status changes during hemodialysis, when potential imbalance is directly attributable to treatment. How serum bicarbonate performs in ESRD patients at estimating acid-base status compared to the gold standard – pH – is unknown.


In a cohort of veterans with ESRD, we prospectively assessed acid-base status monthly at the beginning and end of routine hemodialysis for three months by measuring bicarbonate (as total CO2) and pH. Bias of measured bicarbonate was evaluated with Bland-Altman plots and concordance correlation coefficients comparing measured bicarbonate to that calculated from the Henderson-Hasselbalch equation using pH and pCO2. Categorizing measured bicarbonate and pH, we determined the proportion of pre-dialysis paired samples for which measured bicarbonate misclassified acid-base status (i.e. measured bicarbonate < 24 mmol/L when pH > 7.40, or vice versa).


Among 25 participants, mean pH was 7.41 (± 0.04) pre-dialysis and 7.48 (± 0.05) post-dialysis; 75% of post-dialysis pH measurements were ≥ 7.45. Compared to calculated bicarbonate, measured bicarbonate produced lower estimates of acid-base status both pre-dialysis (mean difference ranged from -1.52 [95% CIs -1.91 to -1.13] to -1.35 [95% CIs -1.69 to -1.00] mmol/L) and post-dialysis (mean difference ranged from -2.70 [95% CIs -3.41 to -1.99] to -0.80 [95% CIs -1.29 to -0.31] mmol/L). There was considerable month-to-month variability in concordance both pre-dialysis (ranging from 0.61 [95% CIs 0.42 to 0.79] to 0.85 [95% CIs 0.76-0.95]) and post-dialysis (0.44 [95% CIs 0.25-0.64] to 0.87 [95% CIs 0.77-0.97]). Compared to pH, 21 of 71 (30%) pre-dialysis bicarbonate samples misclassified acid-base status.


The majority of patients became alkalemic during hemodialysis despite arriving with normal acid base status – a finding currently under recognized in clinical practice. Measured bicarbonate suggested a more acidemic state than was measured by pH. Serum bicarbonate measurements misclassified acid base status in 30% of cases. If confirmed, this observation could impact how physicians prescribe dialysate bicarbonate.


  • Other NIH Support