Value of PAX8 and WT1 Immunostaining in Confirming the Ovarian Origin of Metastatic Carcinoma in Serous Effusion Specimens

We evaluated the detection rates of PAX8 and WT1 immunostaining in 68 (45 as cell blocks, 23 as smears) serous effusion specimens that had a cytologic diagnosis of metastatic carcinoma of ovarian origin. Of the cases, 58 (85%) were positive for PAX8, 56 (82%) were positive for WT1, and 64 (94%) were immunoreactive with either or both markers. Detection rates of PAX8 and WT1 were 85% (44/52) and 92% (48/52), respectively, for metastatic serous carcinoma and 100% (5/5) and 20% (1/5), respectively, for metastatic clear cell carcinoma. Detection rates using cell blocks and smears were 91% and 78%, respectively, with PAX8 and 82% and 83%, respectively, with WT1. We concluded that PAX8 and WT1 had comparable overall detection rates in confirming ovarian origin of malignant effusion. The combination of both markers substantially improved the detection rate. Cell blocks and smears can be used for staining, but a cell block is preferred for PAX8 staining.