A 79-year-old female with productive cough and pleural effusion has pleural fluid with decreased glucose and elevated LDH, and cytology shows malignant cells. What is the most likely cause of the effusion?

• A. Malignancy
• B. Bacterial pneumonia
• C. Heart failure
• D. Pulmonary embolus

Answer: (A)

The situation points to a malignant involvement of the pleura. When pleural fluid analysis shows an exudate pattern with low glucose and high LDH, and malignant cells are present on cytology, this is diagnostic of malignant effusion. Cancer in the pleural space often drives an inflammatory milieu with rapid cell turnover, which lowers glucose as tumor and inflammatory cells consume it, and raises LDH from cell breakdown and increased vascular permeability.

This helps distinguish it from other causes. A parapneumonic effusion from bacterial pneumonia can be exudative and may have low glucose in empyema, but malignant cells would not be present. Heart failure typically causes a transudative effusion with normal glucose and LDH levels and no malignant cells. A pleural effusion from a pulmonary embolus can be exudative and LDH may be elevated, but again malignant cells would not be expected.

So the combination of malignant cells in the pleural fluid along with an exudative profile makes malignancy the most likely cause of the effusion.