title: Accuracy of pleural fluid pH and PCO2 measurement in a blood gas analyser. Analysis of bias and precision.
abstract: 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.
authors.affiliation: Department of Intensive Care, Royal Brisbane Hospital, University of Queensland, Herston, Australia. venkateshb@health.qld.gov.au
authors.forename: B
authors.initials: B
authors.lastname: Venkatesh
authors.forename: R J
authors.initials: RJ
authors.lastname: Boots
authors.forename: S C
authors.initials: SC
authors.lastname: Wallis
mesh_terms: D015374:Biosensing Techniques; D001784:Blood Gas Analysis / Q000592:standards; D002245:Carbon Dioxide / Q000032:analysis*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D010996:Pleural Effusion / Q000737:chemistry*; D011309:Preservation, Biological; D012015:Reference Standards; D012680:Sensitivity and Specificity
chemical_list: D002245:Carbon Dioxide