The detection of cancer cells in the blood can confirm the diagnosis of lung cancer metastasis in the bloodstream, but this test is not routinely performed in clinical practice and is only used in scientific research. Therefore, there are no indicators in blood that can 100% confirm the diagnosis of lung cancer metastasis, but they have suggestive meaning and can be used by experienced lung cancer doctors in combination with other clinical indicators for differentiation. 1.Blood biochemical examination: For primary lung cancer, there is no specific blood biochemical examination. For lung cancer patients, elevated plasma alkaline phosphatase or blood calcium may be considered as bone metastasis; elevated plasma alkaline phosphatase, glutamic oxalacetic transaminase, lactate dehydrogenase or bilirubin may be considered as liver metastasis. (2) Blood tumor markers: There are no specific lung cancer markers for clinical diagnosis, but the following tests can be used as reference for lung cancer assessment: (1) Carcinoembryonic antigen (CEA): At present, CEA in serum is mainly used to determine the prognosis of lung cancer and monitor the treatment process. (2) Neurospecific enolase (NSE): It is the preferred marker for small cell lung cancer and is used for diagnosis and monitoring of treatment response in small cell lung cancer. (3) Cytokeratin fragment 19 (CYFRA21-1): it has some reference significance for the sensitivity and specificity of lung squamous carcinoma diagnosis. (4) Squamous cell carcinoma antigen (SCC): it has some value in monitoring the efficacy and prognosis of lung squamous cell carcinoma.