PURPOSE: Collection of pleural fluid in an arterial blood gas syringe has been promoted as essential for accurate measurement of pH. This study compared the pH of pleural fluid collected by 2 different methods and measured the effect of a 4 hour delay. Design: Prospective observational study of adult patients undergoing thoracentesis. Objective: 1) To compare the pleural fluid pH of samples collected in a heparinized blood gas syringe (BGS) versus Vacuette with lithium-heparin (GTT). 2) To compare the pleural fluid pH on samples obtained in GTT analyzed at 0, 1, 2, and 4 hours.
METHODS: Thoracentesis was performed with a catheter system (Arrow-Clark Pleura-Seal). Pleural fluid was aspirated directly into an evacuated BGS and kept on ice. Additional fluid was drawn into a 50 cc syringe and transferred to GTT held at room temperature. Pearson correlation analysis and intra-class correlation analysis were utilized to assess the agreement between pH-BGS and pH-GTT and agreements of pH at 0, 1, 2, and 4 hours.
RESULTS: Eighty-two patients with mean age of 65±16 years; 49% with associated malignancy. Correlation analysis revealed excellent agreement between BGS and GTT at time zero (Pearson Correlation Coefficient =0.98, p value <0.0001) with a mean bias of -0.03. Correlation analysis revealed a very good agreement between GTT-0 and GTT over time, correlations greater than 0.92. Percent error was less than 1%. Mean bias at 1, 2, and 4 hours were -0.01, -0.01 and 0.01.
CONCLUSION: The pH of pleural fluid collected in an ABG syringe and a Vacuette with lithium heparin does not differ significantly. There is minimal variation of the pH of samples collected in a GTT held at room temperature for up to 4 hours.
CLINICAL IMPLICATIONS: Collecting pleural fluid in a GTT, despite a 4 hour delay of analysis, provides a convenient technique for accurate pH measurement. The requirement that pleural fluid for pH analysis be collected in a blood gas syringe should be discarded.
DISCLOSURE: William Petersen, No Financial Disclosure Information; No Product/Research Disclosure Information