Carcinoembryonic antigen 300 means that the test value of carcinoembryonic antigen is 300, which is far beyond the common reference range of carcinoembryonic antigen, and further examination is needed to exclude tumor diseases. Carcinoembryonic antigen is a tumor marker, commonly used for tumor screening, most of the testing agencies’ reference value is mostly less than 10, some <2.5ug/L, some 0~5ng/mL, 300 is far beyond the reference value of the common testing agencies, and needs to be further investigated for the cause. Elevated carcinoembryonic antigen can be seen in different neoplastic diseases such as gastric cancer, intestinal cancer, breast cancer, pancreatic cancer, gallbladder cancer, etc. It can also be seen in smoking group or non-neoplastic diseases such as pancreatitis, tuberculosis, etc. The greater the elevation, the greater the relationship with neoplastic diseases, but the specificity is low, it can't be sure which kind of tumor it is, and it needs further examination to help determine it, such as CT and colonoscopy. Therefore, carcinoembryonic antigen 300 exceeds the common reference range of carcinoembryonic antigen, and it is recommended that the patient improve the comprehensive examination under the guidance of the doctor to find the cause of the disease.