What CEA is
Carcinoembryonic antigen (CEA) is a protein made by some tumour cells that can be measured in a blood test. In colorectal cancer it is used as a tumour marker: it is not used to diagnose the disease, but to follow it.
The "normal" value depends on the laboratory; most consider below 3-5 ng/mL normal. In smokers the upper limit is usually somewhat higher.
Why your oncologist measures it
- Before surgery: as a baseline. If it was high and falls after the operation, it indicates the tumour has been removed.
- During chemotherapy: if it falls, that usually goes hand in hand with the treatment working; if it rises steadily, it may be a sign that imaging should be reviewed.
- During follow-up: guidelines recommend measuring it every 3-6 months in the first years after treatment, because a rise can flag a relapse before it shows on a CT scan.
Why a high CEA is not always cancer
CEA also rises for reasons that have nothing to do with the tumour:
- Smoking.
- Inflammation or infection: hepatitis, pancreatitis, chronic bronchitis (COPD), inflammatory bowel disease, ulcers.
- Cirrhosis and other liver diseases.
- Benign polyps and some non-cancerous thyroid or breast conditions.
- Laboratory variation: two consecutive readings can differ without anything having happened.
In addition, 20-30 % of colorectal cancers do not produce CEA: in those people the marker can be normal even when the tumour is active. That is why it is never used on its own.
What really matters: the trend
A single value of 6 or 8 ng/mL says little. What the oncologist looks at is whether CEA rises steadily over two or three consecutive tests. When that happens, the usual step is to bring forward an imaging test (CT, MRI or PET-CT) to look for the cause. Treatment decisions are made on the images, not on the number.
What to ask at your appointment
- What was my CEA before surgery or treatment? Did my tumour "produce" CEA?
- Is this rise significant, or within normal variation?
- Are we going to repeat the test or bring forward a scan?
- Is there any other cause (infection, medication, smoking) that could explain the rise?