Pleural effusion: diagnostic value of measurements of PO2, PCO2, and pH

A review of the literature suggests that the measurement of the partial pressure of oxygen (PO2) and carbon dioxide (PCO2) and pH may provide additional diagnostic, therapeutic, and prognostic information in the management of pleural effusions. Parapneumonic effusions with a pH less than 7.2 indicate an impending empyema requiring tube thoracostomy in more than 98% of cases. A distinction between a tuberculous pleural effusion and a malignant pleural effusion of recent onset (less than two months) can frequently be made by measuring the pleural fluid pH. In 100% of reported cases, tuberculous pleural effusions have a pleural fluid pH less than 7.4, whereas over 60% of recent malignant effusions have a pleural fluid pH greater than 7.4. Generally, measurements of PO2 and PCO2 have little discriminatory value in determining cause or proper management of pleural effusions. It is recommended that proper anaerobic collection of pleural fluid for pH measurements be obtained routinely in all pleural effusions of unknown cause.