The goal of our prospective examination was 1) to test the utility of cfDNA isolated from pleural effusion samples in patients with lung disease, including adenocarcinoma, 2) to analyze correlations between pleural effusion and plasma cfDNA amounts and quality, 3) to prove the relationship between pleural fluid cfDNA and sediment cells DNA, 4) to identify and compare relevant pathogenic gene variants (EGFR, KRAS and BRAF) in cfDNA originated from pleural effusion and blood and 5) compare the cfDNA based molecular profile with the tumor tissue and the effusion sediment cell derived gDNA data. The gene discussed is KRAS; the disease is adenocarcinoma.