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

Background: Usually, pleural fluid (PF) pH is measured immediately followin g thoracentesis, and if there is any delay in the measurement, the PF sampl e 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 syri nge was measured immediately following thoracentesis by an arterial blood p H/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 GO-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 st atistically (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 presented at room temperature does not change significantly during the first hour following thoracentesi s. Hence, contrary to the common medical practice, there is no need to perf orm the pll measurement within minutes after thoracentesis and to preserve a pleural fluid sample on ice.