Carcinoembryonic antigen is CEA and its normal or abnormal index does not determine whether one has cancer or not. Unless the CEA value is much higher than the normal value, such as greater than 100ng/mL or 200ng/mL, cancer is highly suspected, but the type and location of cancer cannot be determined. CEA can only be used as a basis for monitoring carcinoembryonic antigen or tumor screening during postoperative follow-up of colon or lung malignant tumors, and if the carcinoembryonic antigen index is high, further colonoscopy and lung CT can be done to check whether there is colon or lung cancer. If the index is high, further colonoscopy and lung CT can be done to check whether there is colon cancer or lung cancer, and other imaging and endoscopic examinations should be done to further exclude cancer.