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 +/- SD, 0.08 +/- 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, -0.15 +/- 0.09; 95% CI, -0.13 to -0.18; P < 0.001) and residual heparin (mean change in pH, -0.02 +/- 0.05; 95% CI, -0.01 to -0.04; P = 0.027). Pleural fluid pH was stable at room temperature for 1 hour and significantly increased at 4 (mean +/- SD, 0.03 +/- 0.07; 95% CI, 0.01 to 0.04; P = 0.003) and 24 hours (0.05 +/- 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.

Figure 1.

Effect of the addition of air on pleural fluid pH. Change in pH from baseline is shown. Mean and 95% confidence intervals (95% CI) are shown. P values represent the paired t test compared with pH of pleural fluid at baseline without added air.

Figure 2.

Effect of the addition of 2% lidocaine on pleural fluid pH. Change in pH from baseline is shown with increasing volumes of lidocaine. Mean and 95% confidence intervals (95% CI) are shown. P values represent the paired t test compared with pH of pleural fluid pH at baseline with no lidocaine added.

Figure 3.

Effect of the addition of heparin on pleural fluid pH. Change in pH from baseline is shown. Mean and 95% confidence intervals (95% CI) are shown. P values represent the paired t test compared with pH of pleural fluid at baseline with heparin added.

Figure 4.

Pleural fluid pH stability over time. Change in pH from baseline is shown at different time points. Mean and 95% confidence intervals are shown (95% CI). P values represent the paired t test compared with pleural fluid pH at Time 0.

Figure 5.

(A–D) Effect of variables on pleural fluid glucose. The graph represents change in individual samples. P values represent the paired t test compared with baseline pleural fluid glucose at Time 0. A single result has been removed from the 0.2-ml 2% lidocaine graph due to aberrant values.