Improving Pleural Fluid pH Accuracy: A Quality Improvement Initiative

Fluid pH remains a standard test when evaluating pleural effusions. Studies recommend using a blood gas analyzer to measure fluid pH immediately after sample collection. Centralization of laboratory services at our institution led to migration of blood gas analysis to a single core laboratory with uncertain impacts on pH results.

Pleural fluid samples were obtained from 30 patients during routine clinical care, excluding those with frank purulence or blood. Fluid pH and pCO2 were measured on a blood gas analyzer. We compared samples collected in plastic screw-top vials from commercially available thoracentesis kits vs paired samples collected in blood gas syringes. We also examined how differences in processing time, fluid pCO2, and heparin anticoagulant correlated with measured pH.

Of the 30 paired samples obtained, 25 (83.3%) were exudates and 5 (16.7%) were transudates. All samples collected in blood gas syringes had lower mean pH and higher mean pCO2 than samples collected in plastic screw-top vials (mean pH difference ± SD, 0.16 ± 0.06; P < 0.001) (mean pCO2 difference ± SD, 14.09 ± 6.92; P < 0.001). Differences in fluid pH were independent of total nucleated cell count (range 65-28 380 cells/mm3). Neither the presence of heparin in the syringe nor time delays up to 2 h between collection and analysis meaningfully impacted the pH result (P = 0.51 and P = 0.75, respectively).

To accurately measure pleural fluid pH, specimens should be collected anaerobically in blood gas syringes rather than the screw-top vials provided in thoracentesis kits.