Comparison ofPleural Fluid pHValues Obtained Using BloodGasMachine, pH Meter, andpHIndicator Strip*

Study purpose: Thepurpose ofthisstudy was tocompare thepleural fluid pHvalues obtained with a bloodgasmachine(pHbg), with a pHmeter(pHmet), andwith a pHindicator strip (pHstrip), todetermine ifthepleural fluid pHmeasured by apHmeteror apHindicator strip was sufficiently accurateforclinical decisions. Methods: Thepleural fluid pHwas determined, within 20minafter being collected anaerobically, by a blood gasmachine(CIBA-Corning model288), pHmeter(Corning pHmeter610A), andpH indicator strip (Baxter Diagnostic) following routine laboratory procedures in50pleural fluids. Pleural fluid pHwas determined inseven additional samples withtheblood gasmachine anda pHmeterat25and37°Crespectively, initially, andafter 30min. Results: Themean pHbg (7.42 ± 0.01) was significantly less thanthemean pHmet (7.58 ± 0.02) orthe mean pHstrip (8.23 ± 0.06). There were significant differences between thepHbg andthe pHmet (p < 0.001), andbetween thepHbg andthepHstrip (p < 0.001). Analysis oftheadditional seven samples demonstrated thatwhenthebloodgasmachine was setat25°C, thepHbg (pHbg = 7.54± 0.02) andthepHmet (7.53 ± 0.01) were almost identical. Conclusion: Whenthepleural fluid pHisgoing tobeusedfordecision making, only thepHvalues provided bythebloodgasmachine are sufficiently accurate. (CHEST 1998; 114:1368-1372)