The pH of pleural fluid removed from the thoracic cavity can suggest possible etiologies. The normal pH is 7.6 secondary to a bicarbonate gradient that occurs between serum and pleural fluid.
The pH for pleural transudates is typically 7.4-7.55. The pH for pleural exudates is typically 7.3-7.45. If the pH is < 7.3 lupus pleuritis, esophageal rupture, tuberculosis, rheumatoid arthritis, and malignant effusion must be considered
This drop in pH from the normal value of 7.6 occurs secondary to a decrease in acid production by pleural fluid cells and a blocked hydrogen efflux in the setting of inflammation, malignancy, and fibrosis