p53 gene product in pleural effusions. Practical use in distinguishing benign from malignant cells

To determine the utility of positive p53 immunostaining as an adjunct in the diagnosis of malignancy in pleural effusions, we reviewed 103 effusions representing the typical range of diagnoses encountered in the evaluation of pleural fluid cytology.

Immunohistochemistry was performed on paraffin-embedded cell blocks using a monoclonal antibody to the p53 suppressor gene product clone BP53-12 and a standard avidin-biotin complex technique with a citrate buffer antigen retrieval solution.

Forty-one of 75 effusions with an unequivocal cytologic diagnosis of malignancy were immunopositive for p53 protein (55%). One of nine effusions cytologically interpreted as showing reactive mesothelial cells showed immunopositivity; that case was subsequently diagnosed as a mesothelioma on pleural biopsy. Nineteen cases were interpreted as suspicious for malignancy. Of these, 16 were negative, and 3 were positive for p53 protein. Of the three positive cases, two showed the presence of non-small cell and poorly differentiated large cell carcinoma.

These findings suggest that p53 protein immunostaining is relatively sensitive and highly specific in differentiating benign mesothelial cells from malignant cells in pleural effusions. While negative p53 protein immunostaining does not exclude malignancy, positive staining in reactive or suspicious cells warrants further diagnostic evaluation of the patient.