Normal carcinoembryonic antigen does not exclude lung cancer. Carcinoembryonic antigen is a protein that is secreted by very few cells in normal adults. If the carcinoembryonic antigen is high and at the same time a clear occupancy is found on the lung, it should indeed be considered as lung cancer, but if the carcinoembryonic antigen is normal, it means that the cells on the occupancy do not secrete this glycoprotein, but may secrete other tumor markers, for example, small cell lung cancer may mainly secrete proteins such as neuron-specific enolase, and squamous lung cancer may secrete squamous cell antigen, etc. There are also some lung cancers Some lung cancers do not even secrete abnormal proteins, but the tumor markers are basically normal, but the final pathological diagnosis is still lung cancer, so the final diagnosis cannot rely on tumor markers to be clear, but still needs to be confirmed by pathology.