Adequacy in pleural effusion: What is the minimum volume required for detection of malignant cells?

Background: Adequacy criteria of pleural fluid volume for optimal reporting are contentious, and very little literature is available to date. This problem has not been addressed in the novel International System for Reporting Serous Fluid Cytology.

Materials And Methods: A retrospective analysis was performed on 939 pleural fluid samples. Five volume bins were created: 0-9.9 ml, 10-19.9 ml, 20-34.9 ml, 35-69.9 ml, and > 70 ml and included 203, 222, 314, 174, and 26 samples, respectively. Volume bins were compared across various categories using a Chi-square test. A malignancy fraction was used to assess diagnostic accuracy. Descriptive statistics for categorical variables were done with median and interquartile range. A ROC curve was constructed to find if pleural fluid volume can be used to detect malignancy. A cut-off volume was found which can detect malignancy with optimum sensitivity.

Results: The area under the Receiver Operating Characteristic curve showed that 55% of the time, the pleural volume can detect malignancy correctly. From the coordinates of the curve it was found that for a sensitivity of 81% and specificity of 40%, a cut-off volume of 13.5 ml of pleural fluid is sufficient to detect malignancy.

Conclusions: We recommend 13.5 ml as the minimum volume cut-off for a satisfactory pleural effusion cytology report. Below this volume, the false-negative rates increase, and the specimen may be deemed as limited for a conclusive diagnosis. As the volume rises above this threshold volume, the false negativity rate decreases but does not significantly improve malignant cells' detection.