CEA is a protein found in blood. Blood levels of CEA are often raised in people with cancer of the bowel. But, testing for CEA isn't a good way of screening for bowel cancer. (Screening means testing to find a cancer at an early stage, before there are any symptoms.)

There are 2 main reasons why the CEA test can't be used to screen for bowel cancer:

It's not sensitive enough

Not everyone with bowel cancer has high levels of CEA. So, a negative CEA test can't rule out bowel cancer.

When CEA levels rise because of a bowel cancer, the cancer has often been developing for sometime. So, measuring CEA levels isn't a good way of detecting bowel cancer early on.

It's not specific enough

CEA levels can be raised by non-cancerous conditions like inflammation, infections and bowel disease. Because of this, many people with a high CEA level don't have cancer. So, if CEA levels were used to screen for cancer many people would have invasive tests they didn't need, such as a colonoscopy.