What is the diagnosis for a pleural effusion with a pH of less than 7?

• A. Simple pleural effusion
• B. Empyema
• C. Pneumonia
• D. Hydrothorax

Answer: (B)

A pleural effusion with a pH of less than 7 is indicative of empyema, which is a collection of pus in the pleural cavity. This low pH reflects an acidic environment often caused by infection and the presence of inflammatory cells and bacteria. In cases of empyema, the pH can drop due to the metabolic activity of bacteria and leukocytes, which produce lactic acid in the setting of inflammation and infection.

In contrast, a simple pleural effusion typically does not exhibit a pH below 7 since it is usually transudative, resulting from conditions like heart failure or cirrhosis, where the pleural fluid is less likely to be infected. Pneumonia may lead to an effusion but does not specifically dictate pH levels, and hydrothorax, like simple pleural effusion, is not associated with a low pleural pH. Thus, the presence of a pH lower than 7 in pleural fluid strongly supports the diagnosis of empyema due to its association with infectious processes within the pleural space.