Background: This is a prospective study to define the volume of pleural fluid adequate for maximal yield of cytologic analysis of pleural fluid.
Methods: Patients undergoing diagnostic thoracentesis with malignancy in the differential diagnosis were enrolled in the study. The first 50 mL of pleural fluid were put in a specimen cup, and subsequent fluid was collected in a drainage bag. Both samples were sent for cytologic evaluation. The cytologist was blinded as to which specimen was being evaluated.
Results: Forty-four patients (21 men, 23 women; mean [+/- SD] age, 46 +/- 11.1 years) were enrolled in the study. The average volume of the "large-volume" specimen was 890 +/- 375 mL (range, 250 to 1,800). Although malignant pleural involvement had never been documented for any patients, 31 patients had received a diagnosis of malignancy prior to undergoing thoracentesis. Cytologic tests were positive for malignancy in 23 of the 44 patients (55%). In the group of patients with an established history of cancer, pleural fluid was positive for malignant cells in 19 of 33 samples (58%). In all 23 patients with malignant pleural effusion, both the 50-mL specimen and the large-volume specimen were cytologically identical. In all 21 patients with negative pleural cytology findings, there was again 100% concordance between the 50-mL samples and the larger samples. The minimum adequate pleural fluid volume for cytologic diagnosis has been a matter of debate. The strongest data to date came from a retrospective study in 2002.
Conclusions: Our prospective study now unequivocally supports the concept that the submission of > 50 mL of pleural fluid for cytologic analysis does not increase diagnostic yield.