Diagnostic significance of pleural fluid pH and PCO₂

The pH, Pco₂ and bicarbonate levels were measured in 178 pleural effusions grouped according to preset criteria into various diagnostic categories. The pH and Pco₂ were found to vary widely, the ranges being 6.80 - 7.60 and 26 - 126 mm Hg, respectively. The effusions associated with pneumonia (parapneumonic effusions) showed the most widely varying values of pH. Apparently the pleural fluid pH has predictive value for the course of parapneumonic effusions. Therapy consisting of antibiotics and thoracentesis alone was inadequate to effect resolution of any of the five parapneumonic effusions with a pH of less than 7.20. None of these 5 effusions was grossly purulent. All 19 parapneumonic effusions with a pH greater than 7.20 resolved with antibiotic therapy, without tube drainage. The mean pH of tuberculous effusions was significantly lower than the mean pH of malignant effusions. When these two possibilities exist, a pH below 7.30 is highly suggestive of tuberculosis, while a pH greater than 7.40 is highly suggestive of malignancy. It is concluded that the measurement of pleural fluid pH is useful in the diagnosis and management of pleural effusions.