Agreement between two plasma bicarbonate assays in critically ill patients

Previous studies have suggested that,measurement of plasma bicarbonate conc entration using the Henderson-Hasselbalch equation may be unreliable, parti cularly in critically ill patients. We examined the agreement between two p lasma bicarbonate concentration assays in critically ill patients. Data wer e collected from records of routine daily blood samples. Paired samples wer e taken at the same time from arterial lines. A Bland-Altman analysis was u sed to compare two bicarbonate assays in clinical use. The first used the H enderson-Hasselbalch equation for blood-gas machine calculations. The secon d used a spectrophotometric enzymatic technique. Comparing the enzymatic me thod to the calculated methods (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 ummol/l to 2.65 mmol.l. This study found poor agreement between the two bic arbonate assays. This poor agreement is clinically important but the causes are unclear. We suggest further investigation of the reliability of bicarb onate assays.