Clinically Important Factors Influencing the Diagnostic Measurement of Pleural Fluid pH and Glucose

Abstract

Rationale: Accurate pleural fluid pH and glucose measurement is a key component in the diagnosis and management of patients with pleural effusion. Standardized methods of pleural fluid collection have not been defined. Objectives: To assess the effect of common clinical factors that may distort measurement accuracy of pleural fluid pH and glucose. Methods: Ninety-two exudative pleural aspirates were collected in commercially available blood gas syringes. Measurements and Main Results: Samples were analyzed immediately using a blood gas analyzer. The effects of residual air, lidocaine, heparin, and delay in analysis (24 h) on pH and glucose measurement accuracy were assessed. Pleural fluid pH was significantly increased by residual air (mean 6 SD, 0.08 6 0.07; 95% confidence interval [CI], 0.06 to 0.09; P , 0.001) and significantly decreased by residual lidocaine (0.2 ml; mean change in pH, 20.15 6 0.09; 95% CI, 20.13 to 20.18; P , 0.001) and residual heparin (mean change in pH, 20.02 6 0.05; 95% CI, 20.01 to 20.04; P 5 0.027). Pleural fluid pH was stable at room temperature for 1 hour and significantly increased at 4 (mean 6 SD, 0.03 6 0.07; 95% CI, 0.01 to 0.04; P 5 0.003) and 24 hours (0.05 6 0.12; 95% CI, 0.03 to 0.08; P , 0.001). Pleural fluid glucose concentration was not clinically significantly altered by residual air, lidocaine (up to 0.4 ml), or 24-hour analysis delay. Conclusions: Accuracy of measured pleural pH is critically dependent on sample collection method. Residual air, lidocaine, and analysis delay significantly alter pH and may impact on clinical management. Pleural fluid glucose concentration is not significantly influenced by these factors. Protocols defining appropriate sampling and analysis methods are needed.

Interpretation of pleural fluid biochemistry

“…Efforts must be made to ensure fluid is collected anaerobically, placed in ice and analysed within 1 hour to minimise alteration of the pH by external factors such as air and pCO2. Similarly Lidocaine can falsely reduce the pleural pH, even with volumes as small as 0.2ml (Rahman et al, 2008). Heparin was also found to lower pleural fluid pH.…”

“…A glucose level of < 3.4mmol/l is sufficiently low to raise concerns of conditions such as complicated parapneumonic effusions, rheumatoid effusions, malignancy and tuberculous effusions (5). Glucose measurements are less vulnerable to the effects of air, Lignocaine and delayed processing (Rahman et al, 2008).…”

Handling Pleural Fluid Samples for Routine Analyses

“…This manoeuvre does not cause a clinically relevant increase in pH, provided care is taken to avoid significant exposure to air (6). Nor is it necessary to pack the fluid sample in ice, as long as the pH determination is performed within 4 hours following extraction (7,8). Ideally, the measurement should be obtained using a blood gas machine.…”

“…Factors influencing pleural fluid pH have less effect on glucose concentration, suggesting that the latter can be used as an acceptable alternative to the former (8).…”

Infection for the acute physician

“…Unfortunately fluid pH may be strongly influenced by sampling errors such as delay in analysis and air or lignocaine in the sampling syringe. 4 Should patients with empyema initially be managed conservatively or surgically? A conservative approach is advocated if sepsis settles promptly with antibiotics, regardless of radiological improvement.…”