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.
All patients undergoing thoracentesis.
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
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 pH0 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 pH0) were not significantly
different from pH0 (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