The AUC of pleural effusion MMP-3, serum MMP-3, and their combination in the differential diagnosis of PEE were 0.764 (95% CI 0.661–0.866), 0.722 (95% CI 0.614–0.831), and 0.810 (95% CI 0.716–0.904), respectively. The gene discussed is MMP3; the disease is Pleural effusion.