Factors influencing the measurement of pleural fluid pH

PURPOSE OF REVIEW

Pleural fluid pH measurement is important in the management of patients with exudative pleural effusions, especially in guiding treatment of parapneumonic effusions. Common variations in the method used to sample pleural fluid affect the accuracy of the value obtained. This article reviews the effects of these variations.

RECENT FINDINGS

Pleural fluid pH is decreased by exposure to acidic fluids, such as retention of local anesthetic or heparin in the syringe or sampling following infiltration of local anesthetic. Exposure of the sample to air leads to an increase in pH. If immediate analysis is not possible, delay of up to 4 h does not cause a significant change in pH, even when the sample is kept at room temperature. It is essential that a blood gas analyzer is used to obtain accurate pH measurement.These factors have less effect on the glucose concentration, which may be used to guide management if an accurate pH value is not available.

SUMMARY

Several common variables in collection method can lead to a clinically significant alteration in the pH value obtained. An evidence-based method for sampling and handling pleural fluid in order to obtain an accurate pH measurement is described.

[Treatment indication with endothoracic drainage tube in parapneumonic effusions by partial pressure of carbon dioxide measurement in pleural fluid]

BACKGROUND AND OBJECTIVE

Parapneumonic effusions with pH < 7.20 or glucose < 0.40 g/l or lactate dehydrogenase (LDH) > 1000 U/l have indication of treatment with endothoracic drainage tube (EDT). The aim of the present study was to determine the accuracy of partial pressure of carbon dioxide (pCO2) measurement in pleural fluid for the subsequent treatment indication with EDT in parapneumonic effusions, by analyzing the area under curve ROC (AUC) and determining the optimal cut off value.

PATIENTS AND METHOD

207 pleural fluids were studied. Glucose, LDH, pCO2, and pH were measured, and data concerning the etiology of pleural effusion and whether EDT treatment was needed were collected after patients were discharged from hospital.

RESULTS

Forty-six out of 207 pleural fluids studied were parapneumonic effusions. Thirty-two were treated with EDT. AUC values were 0.888 (p < 0.0001), 0.890 (p < 0.0001), 0.816 (p < 0.0001), and 0.801 (p < 0.0001) for pCO2, pH, glucose, and LDH, respectively. No significant differences were found among them. Optimal cut off value for pCO2 was 48.6 mmHg, exhibiting 90.6% sensitivity and 78.6% specificity. All parapneumonic effusions showing pCO2 > 60.9 mmHg were treated with EDT. Remarkably, 3 out of 46 parapneumonic effusions (6.5%) that had been improperly treated following pH, glucose or LDH values, were correctly treated following pCO2.

CONCLUSIONS

pCO2 determination in pleural fluid appears to be the best way to decide the indication of EDT in parapneumonic effusions.