Agreement between bicarbonate measured on arterial and venous blood gases.

Objective: This study aims to determine the extent of agreement between venous and arterial bicarbonate for a group of emergency department patients with respiratory or metabolic illness requiring blood gas analysis as part of their evaluation.

Methods: This prospective study of patients who were deemed by their treating doctor to require an arterial blood gas analysis to determine their ventilatory or acid-base status, compared bicarbonate on an arterial and a venous sample taken as close to simultaneously as possible. Data were analysed using bias (Bland-Altman) methods. Subgroup analyses were performed for the metabolic, respiratory, chronic obstructive airways disease and acidotic subgroups.

Results: Two hundred and forty-six patients were entered into the study; 195 with acute respiratory disease and 51 with suspected metabolic derangement. The values of bicarbonate on arterial and venous samples showed close agreement with an average difference between the samples of 1.20 mmol/L (95% limits of agreement being -2.73 to +5.13 mmol/L). Similar agreement was found for all subgroups.

Conclusion: Venous bicarbonate estimation shows a high level of agreement with the arterial value, with acceptably narrow 95% limits of agreement. These results suggest that venous bicarbonate estimation may be an acceptable substitute for arterial measurement.