Pleural fluid cytology yields and time to sample processing

Pleural fluid cytology is an essential component in diagnosing malignant pleural effusions. It is therefore vital to identify the optimal timing for cytologic evaluation to maximize diagnostic yield. This study assesses the yield of pleural fluid cytology based on the length of time between collection of pleural fluid and the start of cytologic processing.

This retrospective analysis included patients who underwent thoracentesis with pleural fluid samples sent for cytology. A Diff-Quik®, ThinPrep®, and cell block for histologic examination were created using standard institutional protocols. Logistic regression and log-risk models were used to assess the association between time to sample processing and malignant diagnosis.

327 pleural fluid samples were included, of which 81 were positive for malignancy. Thoracenteses occurred most often on Tuesdays and Fridays and in the afternoons. There was no relationship between time of day of sample collection and yield, nor between day of the week collected and yield for malignant cells. Furthermore, there was no statistically significant difference in yield based on absolute time between sample collection and processing. This remained true even after adjustment for sample volume.

Despite historical beliefs, our study failed to show a statistically significant association between cytology yield for malignant cells and increasing time from sample collection to laboratory processing. Given these findings, clinicians may be reassured that timing of diagnostic thoracentesis likely does not affect the probability of malignant diagnosis. Other components of sample processing and storage warrant further study in efforts to maximize diagnostic yield.