3 separate thoracentesis can detect up to 90 percent malignant effusions

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Although the first pleural fluid cytologic study can detect malignant cells in up to 60% of effusions, 3 separate thoracenteses can detect up to 90% of malignant effusions.  In theory, the older degenerated cells are largely removed in the earlier thoracenteses, leaving a higher percentage of fresher cells to be detected.  As a result, a repeat thoracentesis should be done to evaluate for malignancy despite the negative cytology seen in the first.

If there is still a suspicion of malignancy despite repeated thoracenteses, thoracoscopy should be performed.  In several studies, thoracoscopic pleural biopsy was found to yield a diagnosis of malignancy in about 70% of patients with pleural effusion of unclear etiology.