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, immu nostains employing various antibodies have improved the diagnostic accuracy of malignant effusions. We investigated the usefulness of Ki67 (MIB1) anti gen immunostaining in the evaluation and diagnosis of malignant serous effu sions. Cell block sections from a total of 54 cases of serous effusions cyt ologically diagnosed as malignant (28), suspicious (6), and benign (20) wer e immunostained with MIB1 monoclonal antibody to the Ki67 nuclear prolifera tion antigen according to the avidin-biotin immunoperoxidase method. The pa tients were 30 women and 24 men with an average age of 58 yr Ki67 (MIB1) im munostain 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%) b enign/reactive. Further investigation revealed histologic, radiologic, and/ or clinical evidence of malignancy in the 3 suspicious (but not in the beni gn/reactive) cases with Ki67 LI values higher than 20%. Correlation between Ki67 LI (>20%) and cytologic effusion type (benign, suspicious, or maligna nt) was statistically significant (P < 0.0001). Ki67 immunostaining has val ue as an adjunct testing to cytomorphology and other immunostains in distin guishing benign from malignant effusions. The addition of Ki67 immunostaini ng to conventional cytology appears more sensitive than cytomorphology alon e and may assist in arriving at accurate diagnoses in suspicious cases with inconclusive cytomorphologic features. (C) 1999 Wiley-Liss, Inc.