Accuracy and reproducibility of pleural effusion cytology

The increasing number of Malignant Mesothelioma (MM) cases that arrive for expert examinations to court for compensation reasons in subjects exposed to asbestos, in many instances rely exclusively on cytological smears of pleural effusion. We evaluated the accuracy and reproducibility of cytological pleural effusions, based on morphological criteria alone. Nine pathologists and eight residents from seven institutions in north-east Italy blindly examined 45 smears of MM (17), metastases (14) and benign effusions (14), in two rounds. Diagnoses had been confirmed by immunohistochemical and clinical follow-up, and eventually at autopsy. Diagnostic accuracy, interobserver and intraobserver agreement in the distinction of benign vs malignant cases, and in the differentiation of primary from metastatic malignancies, were evaluated. The distinction of benign from malignant smears resulted rather satisfactory (k=0.514), but markedly decreased in differentiation of MM from metastases (overall agreement: k=0.343), as well as when readings from residents were analyzed (k=0.132). Cytology is a useful and reliable tool in the identification of malignancies, but when the distinction of primary from metastatic tumors is addressed morphological criteria alone are not sufficient for a definite diagnosis of MM and the use of cell blocks, immunohistochemistry (IHC) and molecular ancillary techniques are recommended.