Pleural effusion smears from 112 patients with either benign or malignant lung disease were investigated for the expression of EGFr and the oncogene proteins myc p64 and ras p21. The streptavidin-biotin peroxidase technique was used. In the studied malignant group of effusions both EGFr and ras have greater sensitivity in the detection of a malignant process than does routine cytological examination though EGFr was less specific. The combination of positive cytology and 3 positive markers is highly specific for a malignant process (90%). Myc and ras had a 100% sensitivity in squamous cell carcinomas but an overall specificity of only 67.3% and 66.6% respectively. The differences in myc and ms positivity, between squamous cell and adenocarcinoma effusion smears were highly significant (p <0.005). All effusion smears associated with undifferentiated carcinomas were ras positive and 2 of them were myc and EGFr positive.

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