Background: The important diagnostic challenge facing the cytopathologist is whether a mesothelial proliferation on effusions represents a malignant mesothelioma (MM) or a benign mesothelial hyperplasia (MH). Here, we evaluated the diagnostic utility of BAP1 immunohistochemistry (IHC) in distinguishing between reactive and neoplastic mesothelial cells. Methods:In pleural and peritoneal effusions from 147 patients with diagnosed MM or with a differential diagnosis of MM and MH, the expression of BAP1 was examined by IHC on paraffinembedded cell blocks (n 5 121) and biopsies (n 5 44). Included were also synchronous and methacronous cytology/biopsy pair samples. BAP1 IHC was evaluated for nuclear staining as positive or negative on target mesothelial cells, with appropriate internal control.Results: In MM cases, loss of BAP1 nuclear staining was observed in 76.5% of the cell blocks and 47.5% of the biopsies. All BAP1-negative cases with a differential diagnosis of benign and malignant mesothelial proliferations were MM at follow-up. All MH cases, the 29% of epithelial MM and the 90% of nonepithelial MM, retained BAP1 expression. Synchronous and methacronous biopsy/cytology pairs showed matching BAP1 results. Conclusion:In effusions with mesotheliomatous cells or atypical mesothelial cells of uncertain significance, negative BAP1 IHC strongly supports a diagnosis of MM. With prudence in interpreting immunostaining, BAP1 may be included in IHC panels for MM cytodiagnosis, given its high specificity and sensitivity. 1 An effusion is the only available sample in specific settings, as in the elderly or in patients with co-morbidities. Based on cytomorphology alone, the differential diagnosis between MM, mesothelial hyperplasia (MH), and metastatic carcinoma may be difficult. It is necessary to use ancillary techniques, primarily immunohistochemistry (IHC). 1 IHC markers to distinguish epithelioid MM from metastatic carcinoma are well-defined, 2,3 whereas those to distinguish epithelioid MM from MH are less definite, except for GLUT-1, IMP-3, Desmin, and EMA. Regarding the practical utility of the latter two markers, Desmin shows low sensitivity 4 and EMA shows low specificity. 5-7Recently, a new marker has been proposed to aid in distinguishing MM from MH, namely BRCA1-associated protein 1 (BAP1 The aim of this study was to evaluate the utility of BAP1 IHC in the diagnosis of MM in effusions. To do this, we investigated the BAP1 protein expression on a large series of cell blocks from pleural/peritoneal effusions, with a definite diagnosis of MM or an uncertain diagnosis between MM and MH, and benign effusions. BAP1 immunostaining was also performed on biopsy samples with the same diagnoses to evaluate the differences between effusion and tissue samples. | M A TE RI A L S A ND M E TH ODSThis study was designated as exempt by the ethics committee due to its retrospective nature; moreover, no protected health information The following formulae of BAP1 testing for the outcome MM were calculated: sensitivity, defined as ...