27. Which of the following statements is true regarding carcinoembryonic antigen (CEA）？
A.CEA is an accurate sereening test for primary colorectal cancer
B. CEA levels have not been helpful in the diagnosis of recurrent colorectal cancer
C. CEA levels, when elevated, are highly specific for colon cancer
D. CEA is present in normal colonic mucosa
E. Postoperative CEA assay is 70% accurate in predicting the appearance of liver metastases within I year

CEA is a protein that is produced by many types of cells, including those in the colon. It is often elevated in people with colorectal cancer. However, CEA can also be elevated in people with other conditions, such as liver disease or pancreatitis. Therefore, CEA is not a specific test for colorectal cancer.

CEA levels can be used to monitor people who have been treated for colorectal cancer. If CEA levels rise after treatment, it may be a sign that the cancer has returned.

Postoperative CEA assay is 70% accurate in predicting the appearance of liver metastases within I year. This means that if CEA levels are elevated after surgery, there is a 70% chance that the cancer has spread to the liver.

A, B, and C are incorrect because CEA is not an accurate screening test for primary colorectal cancer, CEA levels have been helpful in the diagnosis of recurrent colorectal cancer, and CEA levels are not highly specific for colon cancer. D is incorrect because CEA is not present in normal colonic mucosa.