Measuring pleural fluid pH: high correlation of a handheld unit to a traditional tabletop blood gas analyzer

STUDY PURPOSES: To survey hospital laboratories in the United States to determine methods used for measuring pleural fluid pH, and to compare pleural fluid pH values obtained with a traditional tabletop blood gas analyzer (BGA) to those obtained with a handheld analyzer.

Hospital laboratories nationwide were contacted by telephone to survey the methods used to measure pleural fluid pH. In a second phase, pleural fluid was prospectively collected from 19 pediatric and adult patients with pleural effusions, and pleural fluid pH was measured simultaneously with a traditional tabletop BGA and with a handheld unit.

A total of 220 hospital laboratories were contacted by telephone, and 166 responded (75%). The methods for determining pleural fluid pH for all hospital laboratories were pH meter (35%; n = 59), BGA (32%; n = 53), and litmus paper (31%: n = 51); 2% (n = 3) did not perform the test. University hospitals were more likely to use a BGA, compared to community hospitals (p < 0.014) or children's hospitals (p < 0.001). In the comparison of pleural fluid measurements, the mean pH for the traditional BGA was 7.358 +/- 0.189, and the mean pH for the handheld unit was 7.382 +/- 0.203. The absolute difference between the two machines was 0.024 U, and the two methods were correlated (p < 0.01; r = 0.993; degrees of freedom = 36).

Most hospital laboratories in the United States do not measure pleural fluid pH using a traditional BGA and use alternative methods that have previously been shown to be inaccurate. Pleural fluid pH obtained by a handheld unit has a high degree of correlation to that of a traditional tabletop BGA, and it offers a satisfactory alternative for laboratories reluctant to measure pleural fluid pH with a BGA.