The value of epithelial membrane antigen expression in separating benign mesothelial proliferation from malignant mesothelioma: a comparative study.

Differentiating reactive mesothelial (RM) proliferation from malignant mesothelioma (MM) can be cytologically challenging. There have been discordant studies reporting the value of epithelial membrane antigen (EMA) in differentiating RM from MM. In this study, we investigated the expression of two different clones of EMA in RM and MM. Twenty cases of pleural effusion smears of RM and 20 cases of MM with their corresponding cell blocks were retrieved from the hospital computer system. Diagnosis of MM was confirmed by surgical decortication or pneumonectomy with immunostaining studies and/or electron microscopy. Cases of RM were confirmed by clinical history and histology. Cell blocks were formalin-fixed, paraffin-embedded, and immunostained for EMA clone Mc5 and EMA clone E29. The positive rates for clone Mc5 were 14/20 (70%) for MM and 12/20 (60%) for RM and EMA clone E29 were 15/20 (75%) for MM and 0/20 (0%) for RM. The sensitivity and specificity for EMA clone Mc5 were 70 and 40%, respectively. For EMA clone E29, the sensitivity and specificity were 75 and 100%, respectively. In conclusion, both RM and MM immunostained for EMA clone Mc5, indicating that it is not a reliable immunocytochemical marker for differentiating RM from MM. EMA clone E29 was negative in all cases of RM and positive in 75% of MM and therefore is a reliable immunocytochemical marker for differentiating RM from MM.