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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.

Previous studies have suggested that measurement of plasma bicarbonate concentration using the Henderson-Hasselbalch equation may be unreliable, particularly in critically ill patients. We examined the agreement between two plasma bicarbonate concentration assays in critically ill patients. Data were collected from records of routine daily blood samples. Paired samples were taken at the same time from arterial lines. A Bland-Altman analysis was used to compare two bicarbonate assays in clinical use. The first used the Henderson-Hasselbalch equation for blood-gas machine calculations. The second used a spectrophotometric enzymatic technique. Comparing the enzymatic method to the calculated method (enzymatic minus calculated) the bias was -1.6 mmol/l (95% CI: -1.2 to -2.0 mmol/l). The limits of agreement were -5.85 mmol/l to 2.65 mmol/l. This study found poor agreement between the two bicarbonate assays. This poor agreement is clinically important but the causes are unclear. We suggest further investigation of the reliability of bicarbonate assays.

This paper is a brief report of the symposium, “Improving the Evidence Base for Anaesthesia and Intensive Care”, organized by the MASTER Anaesthesia Trial Study Group at the Annual Scientific Meeting of the Australian and New Zealand College of Anaesthetists, Newcastle, N.S.W., on Tuesday, May 5, 1998.