Comparison of Pleural Fluid pH Values Obtained Using Blood Gas Machine, pH Meter, and pH Indicator Strip*

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Study purpose

The purpose of this study was to compare the pleural fluid pH values obtained with a blood gas machine (pHbg), with a pH meter (pHmet), and with a pH indicator strip (pHstrip), to determine if the pleural fluid pH measured by a pH meter or a pH indicator strip was sufficiently accurate for clinical decisions.


The pleural fluid pH was determined, within 20 min after being collected anaerobically, by a blood gas machine (CIBA-Corning model 288), pH meter (Corning pH meter 610A), and pH indicator strip (Baxter Diagnostic) following routine laboratory procedures in 50 pleural fluids. Pleural fluid pH was determined in seven additional samples with the blood gas machine and a pH meter at 25 and 37[degree sign]C respectively, initially, and after 30 min.


The mean pHbg (7.42 +/- 0.01) was significantly less than the mean pHmet (7.58 +/- 0.02) or the mean pHstrip (8.23 +/- 0.06). There were significant differences between the pHbg and the pHmet (p < 0.001), and between the pHbg and the pHstrip (p < 0.001). Analysis of the additional seven samples demonstrated that when the blood gas machine was set at 25[degree sign]C, the pHbg (pHbg = 7.54 +/- 0.02) and the pHmet (7.53 +/- 0.01) were almost identical.


When the pleural fluid pH is going to be used for decision making, only the pH values provided by the blood gas machine are sufficiently accurate.

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