Value of Ki67 immunostain in identification of malignancy in serous effusions

The distinction between malignant and benign serous effusions continues to be a challenging and a frequent problem to cytopathologists. Recently, immunostains employing various antibodies have improved the diagnostic accuracy of malignant effusions. We investigated the usefulness of Ki67 (MIB1) antigen immunostaining in the evaluation and diagnosis of malignant serous effusions. Cell block sections from a total of 54 cases of serous effusions cytologically diagnosed as malignant (28), suspicious (6), and benign (20) were immunostained with MIB1 monoclonal antibody to the Ki67 nuclear proliferation antigen according to the avidin-biotin immunoperoxidase method. The patients were 30 women and 24 men with an average age of 58 yr. Ki67 (MIB1) immunostain labeling index (LI) values were higher than 20% in 23 of 28 (82%) cytologically malignant, in 3 of 6 (50%) suspicious, and in 1 of 20 (5%) benign/reactive. Further investigation revealed histologic, radiologic, and/or clinical evidence of malignancy in the 3 suspicious (but not in the benign/reactive) cases with Ki67 LI values higher than 20%. Correlation between Ki67 LI (> 20%) and cytologic effusion type (benign, suspicious, or malignant) was statistically significant (P < 0.0001). Ki67 immunostaining has value as an adjunct testing to cytomorphology and other immunostains in distinguishing benign from malignant effusions. The addition of Ki67 immunostaining to conventional cytology appears more sensitive than cytomorphology alone and may assist in arriving at accurate diagnoses in suspicious cases with inconclusive cytomorphologic features.