The value of cells in the pleural fluid in the differential diagnosis.

Fifty samples of pleural fluid, collected from consecutive patients in a thoracic clinic who had diagnostic thoracentesis, were studied prospectively. Pleural fluid protein was of value in differentiating transudates from exudates. Pleural fluid red cell counts, white blood cell counts, and differential white blood cell counts have no specificity and no usefulness in the differential diagnosis of the origin of the effusion. Pleural fluid cytology was positive in 60% of all the malignancies studied in this series; for the group with metastatic breast carcinoma, there was a 78% positive pleural fluid cytology. Differential white cell counts revealed tumor cells in 45% of malignant effusions. In our experience, the finding of tumor cells is the only useful finding in differential cell counts of the pleural fluid.