Carbonic anhydrase IX (CAIX) does not differentiate between benign and malignant mesothelium.

To examine carbonic anhydrase IX (CAIX), a marker of renal cell carcinoma that recently has been described in malignant effusions. Pleural and peritoneal fluids with the following diagnoses-reactive (n = 23), carcinoma (n = 17), and "suspicious for mesothelioma" (n = 4)-were immunostained for CAIX, calretinin, Ber-EP4, and MOC31. A tissue microarray of epithelioid (n = 27) and sarcomatoid (n = 8) mesotheliomas and three cases of benign mesothelium were also immunostained for CAIX. Mesothelial cells in both reactive (18/23) and malignant effusions (18/21) were positive for CAIX (P > .05). In carcinomatous effusions, CAIX expression was restricted to the mesothelial cells. Agreement between CAIX and calretinin expression was present in 89% of cases. In tissues, CAIX was positive in 100% of benign and 91% of malignant mesothelium. CAIX can be a useful ancillary marker for identifying mesothelial cells. There is no difference in CAIX expression between benign and malignant mesothelium. Caution should be exercised while evaluating for metastasis from renal cell carcinoma.