Is direct collection of pleural fluid into a heparinized syringe important for determination of pleural pH? A brief report.

INTRODUCTION: It has long been believed that pleural fluid must be directly aspirated into a heparinized syringe to obtain an accurate value. Many operators aspirate 30 to 60 mL of pleural fluid into a syringe without heparin, and then place 1 mL into a heparinized syringe from which the pH is determined. We postulated that this technique does not cause a clinically significant difference in pleural pH values.
METHODS: Patients undergoing thoracentesis in the outpatient clinic, general ward, and medical ICU were eligible. After the initial entry of the needle into the pleural space, a heparinized syringe was used to obtain pleural fluid for pH determination. A 60-mL syringe was then used to aspirate additional pleural fluid for biochemical analysis and culture. At the end of the procedure, a second aliquot of pleural fluid was placed into a heparinized syringe for pH determination. A difference of 0.1 in pH was taken as clinically important.
RESULTS: Twenty-one pleural fluid samples were obtained from 20 patients. Pleural fluid pH determinations were within 0.1 in all but one patient. The mean pH for the directly collected group was 7.39 (25%: 7.35; 75%: 7.45). The mean for the indirectly collected group was 7.41 (25%: 7.35; 75%: 7.45). The difference between the two means (0.02; 95% confidence interval, 0.0368 to 0.00131) was statistically significant but clinically unimportant (p=0.037).
CONCLUSIONS: Pleural fluid can be collected in a large syringe and then placed into a heparinized syringe to assess pH. This is useful information because the use of just one syringe saves time and reduces the risk of iatrogenic complications.