Even though cells drawn from a pleural effusion stain positive for calretinin and negative for CEA that doesn’t mean the person has mesothelioma. Those mesothelial cells may be non-malignant yet still react to immunohistochemical stains used to identify malignant mesothelioma.

Is there a way to distinguish between malignant and merely reactive cells? According to a new paper in Cancer Cytopathology, "The Use of Immunohistochemistry to Distinguish Reactive Mesothelial Cells From Malignant Mesothelioma in Cytologic Effusions", a positive epithelial membrane antigen (EMA) stain coupled with a negative desmin stain strongly points to mesothelioma while the opposite pattern strongly suggests reactive mesothelial cells and not mesothelioma.