CEA is a very important disease monitoring indicator for tumors of gastrointestinal origin and lung adenocarcinoma, but it should be noted that it is not a decisive diagnostic indicator, but can only suggest the diagnosis and detect the disease efficacy. The reference value of CEA varies from hospital to hospital, but most of them are within the range of 0-10μg/L. More than 10μg/L means abnormal, but the reference value of CEA can be above 100μg/L. If it is slightly elevated, such as around 10-20μg/L is not clinically significant. Mainly, further examination should be performed after CEA elevation, such as gastroscopy and colonoscopy of the digestive tract to exclude gastrointestinal tumors, whole abdomen CT scan and enhancement to find out if there is a local mass, or lung CT to find out if there is a mass in the lung. Then further examination should be done for the lung mass and abdominal mass to confirm whether there is a tumor or not.