Comparison of three methods to estimate plasma bicarbonate in critically ill patients: Henderson-Hasselbalch, enzymatic, and strong-ion-gap.
We have previously found poor agreement between Henderson-Hasselbalch and enzymatic methods for estimating plasma bicarbonate concentration in critically ill patients. In this study we compared these two established methods with a new method for estimating bicarbonate using the strong-ion-gap equation. The strong-ion-gap is derived from the Stewart approach to acid-base physiology. One hundred data sets were collected from records of routine daily blood samples in critically ill patients. Bland-Altman analyses were used to compare the three methods. We proposed that bias greater than +/- 1 mmol/l and limits of agreement wider than bias +/- 2 mmol/l were clinically important, Comparing the Henderson-Hasselbalch method to the enzymatic method, the bias was 2.1 mmol/l and the limits of agreement were -1.8 mmol/l to 5.9 mmol/l. Comparing the Henderson-Hasselbalch method to the strong-ion-gap method, the bias was -9.1 mmol/l and the limits of agreement were -17.1 mmol/l to -1.1 mmol/l. Comparing the enzymatic to the strong-ion-gap method, the bias was -11.2 mmol/l and the limits of agreement were -18.2 mmol/l to -4.2 mmol/l. This study found poor agreement between the two established bicarbonate assays and worse agreement between the established assays and the strong-ion-gap method. The strong-ion-gap method is currently too inaccurate for clinical application, but may have future use.
Story
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DA
Department of Anaesthesia, Austin and Repatriation Medical Centre, University of Melbourne, Victoria.
Poustie
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S
Bellomo
R
R
D000137:Acid-Base Imbalance / Q000097:blood; D001639:Bicarbonates / Q000097:blood*; D001784:Blood Gas Analysis / Q000379:methods*; D002245:Carbon Dioxide / Q000097:blood*; D016638:Critical Illness; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007362:Intensive Care Units; D011446:Prospective Studies; D013053:Spectrophotometry
D001639:Bicarbonates; D002245:Carbon Dioxide