Pleural fluid pH: diagnostic, therapeutic, and prognostic value.

Measurement of pleural fluid pH has diagnostic, therapeutic, and prognostic implications in exudative pleural effusions (Table II). A parapneumonic effusion with a pleural fluid pH below 7.2 indicates an empyema is forming which necessitates chest tube drainage in all patients, whereas a pleural fluid pH over 7.3 does not require drainage. If the pH of a parapneumonic effusion is 7.2 to 7.3, serial pleural fluid pH measurements with clinical observation will help to determine the need for chest tube drainage. A pleural fluid glucose level of below 60 mg/dl and a lactic dehydrogenase level over 1,000 IU/dl in conjunction with a pleural fluid pH of 7.2 to 7.3 indicate an impending empyema. These findings are consistent with our clinical experience in patients with parapneumonic effusion. Tuberculous pleural effusions had a pleural fluid pH below 7.4 in all reported patients. This pH may be of value in distinguishing tuberculous pleural effusions from recent malignant effusions, which tend to have a higher pleural fluid pH, particularly if used in conjunction with other pleural fluid values, cell counts, and other clinical parameters. In patients with malignant pleural effusions, a pH of less than 7.3 is usually seen in those effusions present for several months and is associated with a lower glucose level and a higher white cell count and lactic dehydrogenase level. Results of cytologic study of the pleural fluid and pleural biopsy are often positive, there is poor response to sclerosing agents, and the prognosis is poor. A rheumatoid pleural effusion most often has a pleural fluid pH below 7.3. A pleural fluid pH below 6 is seen almost exclusively in esophageal rupture but rarely with empyemas, whereas a pleural fluid pH below 7 occurs in esophageal rupture, empyema, and rheumatoid pleural effusions. In pleural effusions secondary to congestive heart failure, the pH is almost always greater than 7.4 unless systemic acidemia coexists, in which case the pleural fluid pH is within 0.04 units of the simultaneous arterial pH. The major value of pleural fluid pH is to determine the need for chest tube drainage in parapneumonic effusions and to determine the response to sclerosing agents in patients with malignant pleural effusions. As with all diagnostic tests, the results should be interpreted in the context of other diagnostic tests of the pleural fluid and clinical aspects before diagnostic or therapeutic decisions are made.