Based on the close relationship between SHOX2 and RASSF1A gene methylation and the occurrence of lung cancer, and MPE is mostly secondary to malignant tumors from other sources, mainly lung cancer, it is not clear whether SHOX2 and RASSF1A gene methylation status in pleural effusion, whether their methylation is related to malignant pleural effusion, and whether gene methylation in pleural effusion combined with tumor marker CEA detection can improve the diagnostic efficiency of malignant pleural effusion. Here, CEACAM5 is linked to lung cancer.