Main relevant tumors: broad-spectrum tumor markers. Other related tumors: Commonly found in lung cancer, colorectal cancer, pancreatic cancer, gastric cancer, breast cancer, medullary thyroid cancer, etc. Other influencing factors: False positives are more frequent in smokers, and serum CEA is also elevated in about 15-53% of women during pregnancy and in patients with cardiovascular disease, diabetes, and nonspecific colitis. CEA is an important tumor-associated antigen. 70-90% of patients with colon adenocarcinoma are highly positive for CEA, and the order of positivity among other malignancies is gastric cancer (60-90%), pancreatic cancer (70-80%), small intestine adenocarcinoma (60-83%), lung cancer (56-80%), liver cancer (62-75%), breast cancer (40-68%), urological cancer (31-46%) The rate of CEA in gastric fluid (gastric cancer), saliva (oral cancer, nasopharyngeal cancer) and thoracoabdominal fluid (lung cancer, liver cancer) is higher because CEA in these tumor “soaking fluid” may exist before blood. When liver metastasis occurs, the increase of CEA is especially obvious. CEA measurement is mainly used to guide the treatment and follow-up of various tumors, and the continuous observation of CEA concentration in blood or other body fluids of tumor patients can provide an important basis for disease judgment, prognosis and efficacy observation. A large number of clinical practices have confirmed that the preoperative or pre-treatment CEA concentration can clearly predict the status of tumor, survival period and whether there is an indication for surgery. The lower the preoperative CEA concentration, the earlier the stage of disease, the smaller the possibility of metastasis and recurrence, and the longer the survival time; conversely, the higher the preoperative CEA concentration, the more advanced the stage of disease, the more difficult to resect, and the poor prognosis. When surgical resection of malignant tumor is performed, continuous measurement of CEA will help to observe the efficacy. The CEA concentration can also reflect the efficacy of radiotherapy and chemotherapy. As long as the CEA concentration decreases with treatment, it is effective; if the concentration remains the same or even increases with treatment, the treatment plan should be changed. CEA testing can also be used to monitor recurrence and metastasis in patients whose CEA has been normalized by surgery or other treatment methods, with long-term follow-up. The following protocol is usually used: once in the sixth week after surgery; once a month for three years after surgery; once every three months for 3-5 years; once every six months for 5-7 years; and once a year after 7 years. If an elevation is found, it is measured again in two weeks, and both times are indicative of recurrence and metastasis.