Factors that influence Pleural Fluid pH after Thoracentesis

by Joseph Esherick, M.D., FAAFP, FHM

Pleural fluid pH is an important measure for determining the clinical management of pleural diseases. For example, a pleural fluid pH <7.2 in patients with suspected pleural infection should mandate the placement of a chest tube. However, pleural fluid pH might vary based on the techniques used to obtain and test the sample. Using 92 samples from 81 patients, researchers in the U.K. explored the potential effects of time to analysis and of air, lidocaine, or heparin left in the testing syringe.

The presence of air in the syringe increased pleural fluid pH by 0.08 (P<0.001). Residual lidocaine, even at minimal volume, decreased pH (e.g., 0.2 mL of lidocaine lowered pH by 0.15; P<0.001); at larger volumes, the decrease was greater. Retaining the heparin preloaded in the analysis syringe caused a decline of 0.02 (P<0.027). No change from baseline pH value was seen when specimens were analyzed within 1 hour, but pH increased by 0.03 at 4 hours (P=0.07) and by 0.05 at 24 hours (P<0.001). 

Therefore, to obtain an accurate pleural fluid pH, remove all residual air, lidocaine, and heparin from the syringe submitted for pleural fluid pH analysis and analyze the fluid within one hour of collection

Rahman NM et al. Clinically important factors influencing the diagnostic measurement of pleural fluid pH and glucose. Am J Respir Crit Care Med 2008 Sep 1; 178:483.