This is a prospective study to define the volume of pleural fluid adequate for maximal yield of cytologic analysis of pleural fluid.
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.
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.
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.