Accuracy of pleural fluid pH and PCO2 measurement in a blood gas analyser. Analysis of bias and precision.

Pleural fluid pH measurements are used in the management of parapneumonic effusions. This study evaluated the accuracy of the pleural fluid pH and PCO2 measurement in a blood gas analyser (BGA) and the error induced by altering the aerobic environment and temperature of sample storage prior to analysis. The pH electrodes in a pH meter and a BGA were initially evaluated using aqueous reference solutions. The accuracy of pH and PCO2 electrodes was then assessed in pleural fluid tonometered to a known PCO2. The effects of aerobic compared to anaerobic storage and storage temperature, 0 degrees C vs 37 degrees C of pleural fluid were evaluated for errors in pH and PCO2 measurement. The BGA and the pH meter measured pleural pH with negligible bias and imprecision (0.01+/-0.01 pH units). The PCO2 measurement bias and imprecision of the BGA was small: 1.8+/-0.57 torr (0.2+/-0.08 kPa) at 2% CO2, 0.1+/-0.93 torr (-0.01+/-0.1 kPa) at 5% CO2 and -3.77+/-0.81 (-0.5+/-0.1 kPa) torr at 10% CO2 level. Aerobic storage resulted in a clinically important overestimation of pleural fluid pH by both the pH meter and the BGA of 0.14-0.16 pH units (p<0.05). No significant differences were detected when storage at 0 degrees C was compared to that at 37 degrees C. A BGA is reliable for pleural fluid pH measurements. Pleural fluid PCO2 measurements in a BGA have a concentration-dependent bias and imprecision. Anaerobic storage of specimens for pleural pH and PCO2 analysis is mandatory.