Is pH paper an acceptable, low-cost alternative to the blood gas analyzer for determining pleural fluid pH?

Our laboratory uses pH paper rather than a blood gas analyzer to measure pleural fluid pH to decrease cost and avoid analyzer malfunction due to viscous fluids.

To compare these two methods of determining pleural fluid pH, 42 patients undergoing diagnostic or therapeutic thoracentesis had two 1-mL aliquots of pleural fluid anaerobically collected in a heparinized syringe and placed on ice. pH measurements were made using litmus paper (pHydron Vivid 6-8 brand litmus paper; MicroEssential Labs; Brooklyn, NY) and the model 995-Hb blood gas analyzer (AVL Instruments; Roswell, GA) within 1 h of collection. Agreement analysis was performed in three ways: on the entire group; in subcategories of complicated or uncomplicated parapneumonic effusions (<7.1, 7.1 to 7.3, >7.3); and in subcategories of poor prognosis or better prognosis malignant effusions(<7.3, >7.3).

pH measured with pH paper was significantly more variable (SD=0.55, coefficient of variation [CV]=7.5%) than was pH measured with the blood gas analyzer (SD=0.11, CV=1.5%). There was no significant correlation between values obtained with the two techniques (r=-0.26, SD of the differences=0.59). Using the pH subcategories, there was 72% discordance in classification between litmus paper and arterial blood gas (ABG) determinations for patients with parapneumonic effusions. In patients with malignant effusions, there was 30% discordance. The pH values obtained by the ABG analyzer predicted tube thoracostomy 72% of the time, whereas the pH values obtained using pH paper were consistent only 36% of the time.

Determination of pleural fluid pH using pH paper is unreliable and should not be considered an acceptable alternative to the blood gas analyzer. There is no need to determine pH on purulent samples. Hospital laboratories will be more likely to allow the use of the ABG analyzer on fluids other than blood if clinicians keep this in mind.