Does pleural fluid pH change significantly at room temperature during the first hour following thoracentesis?

Headnote

Does Pleural Fluid pH Change Significantly at Room Temperature During the First Hour Following Thoracentesis?*

Background: Usually, pleural fluid (PF) pH is measured immediately following thoracentesis, and if there is any delay in the measurement, the PF sample is preserved on ice.

Objective: To determine if PF pH changes significantly at room temperature during the first hour following thoracentesis.

Design: Prospective, self controlled.

Setting: Tertiary care center.

Patients: All patients undergoing thoracentesis.

Measurements: The PF pH of a sample collected in an arterial blood gas syringe was measured immediately following thoracentesis by an arterial blood pH/gas analyzer. Additional measurements were made at 5, 15, 30, 45, and 60 min from the first pH measurement (pH0), maintained at room temperature.

Results: For 28 PF samples, pH0 (mean +/- SD) was 7.351 +/- 0.158, and the 60-min pH (pH60) was 7.359 +/- 0.161. The mean difference between pH60 and pHO was 0.008 +/- 0.026, which was not significant, either clinically or statistically (p = 0.13). Similarly, the interim pH values (for measures at 5, 15, 30, 45 min after pHO) were not significantly different from pHO (mean differences, 0.002, 0.003, 0.005, and 0.004, respectively; p values, 0.51, 0.21, 0.06, and 0.22, respectively).

Conclusions: The pleural fluid pH of a sample preserved at room temperature does not change significantly during the first hour following thoracentesis. Hence, contrary to the common medical practice, there is no need to perform the pH measurement within minutes after thoracentesis and to preserve a pleural fluid sample on ice. (CHEST 2000; 117:1043-1048)

Key words: empyema; exudate; hydrogen ion concentration; pleural effusion; pleural fluid; specimen handling; temperature; time factor; transudate

Abbreviations: LDH = lactate dehydrogenase; NS = not significant; PF = pleural fluid; pHO = the first PF pH measurement; pH5 = pH measurement made 5 min after the first measurement; pHl5 = pH measurement made 15 min after the frost measurement; pH30 = pH measurement made 30 min after tha first measurement; pH30 = pH measurement made 30 min after tle first measurement; pH45 pH measurement made 45 min after the first measurement; pH60 = pH measurement made 60 min after the first measurement

Multiple tests are performed on pleural fluid (PF) samples obtained by diagnostic thoracentesis to help manage patients with...