One of the easiest ways to track certain cancers is through a simple blood test called a tumor marker test. The presence of certain cancers can be confirmed by the specific proteins being shed into the blood. These proteins are the tumor markers. Carcinoembryonic antigen (CEA) is the protein linked to colorectal cancers.
Every one of us has a little CEA in our blood. The presence of this protein in the blood alone does not mean you have colon cancer. The CEA blood test cannot be used to diagnose cancer, only to track cancer after a diagnosis. Your doctor probably checked your blood for this antigen when you were diagnosed. If elevated amounts of CEA were found in your blood at that time, your doctor can use it to monitor your response to treatment and watch for recurrence with future blood tests.
False positive results -- results that are very high but do not indicate cancer growth -- can occur with CEA. One the main causes for inaccurately high readings is chemotherapy treatment. During chemotherapy, the drugs are supposed to target and kill the cancer cells. As these cells die, CEA is released into the bloodstream and can stay elevated for a few weeks following treatment.
Many Types of Tumor Markers
CEA levels may be elevated and signify a recurrence of cancer or a spread (metastasis) to another site in your body. The levels are considered elevated when they show above 3 nanograms per milliliter (ng/ml) and are very high when they are above 5 ng/ml. Aside from showing growth or resistance of a colorectal cancer, very high CEA levels can also be used to look for cancer or metastasis of the:
However, some of these cancers have more specific tumor markers associated with them. For instance, the marker alpha fetoprotein (AFP) is linked to liver cancer and the marker prostate specific antigen (PSA) is more closely identified with prostate cancer. Your doctor does not use the tumor marker alone -- he or she will pair your blood test results with other diagnostic tools, such as your overall health, response to treatment, and imaging tests, such as the computed tomography (CT) exam or magnetic resonance imaging (MRI) results.
Another potential marker, called the epidermal growth factor receptor (EGFR), is present on every cell and helps them to grow. Some cancers exhibit highly functioning EGFR, which means they may be susceptible to different treatment modalities.
You may also have heard of something called a biomarker for colon cancer tumors. One biomarker gaining popularity is the KRAS gene; according to the National Comprehensive Cancer Network, 40% of people with colon cancer will have a KRAS gene mutation. The doctor can test the tissue from your colon tumor for this mutation, which may help the doctor tailor your treatment plan to your specific type of colon cancer.
CA 19-9 was recognized as a tumor marker for colorectal cancers in the past, but is now more significant for pancreatic and other digestive cancers. However, your doctor may still choose to test your blood for elevated CA 19-9 levels along with CEA testing.
Getting the Test
You do not have to prepare or fast prior to this blood test. On the day of your test, a small blood sample will be drawn from one of the large veins in your arm or hand. If possible, it's a good idea to have the same laboratory perform your repeat CEA tests, since the measurement and tools may vary from lab to lab. Once your doctor receives the CEA results, he or she can discuss them with you and explain how they may (or may not) affect your treatment plan.
Aside from cancer, tumor markers can be elevated for a number of reasons. The American Cancer Society cites that people who smoke or have pancreatitis, chronic obstructive pulmonary disease (COPD) or even hepatitis could have elevated CEA levels without the presence of cancer.
You may have sequential tumor marker tests, especially if you had elevated CEA levels upon being diagnosed with cancer. If the CEA levels drop following successful chemotherapy or bowel surgery, then spike again in the future, it may be an indication that the cancer recurred. A series of test results are more illustrative than one singular result.
American Cancer Society. (2006). American Cancer Society's Complete Guide to Colorectal Cancer. Clifton Fields, NE: American Cancer Society.
American Cancer Society. (n.d.). How are Tumor Markers Used? Accessed June 17, 2012.
Lano, Charlene. (November, 2009). EGFR Blockers & Colorectal Cancer: KRAS Testing Confirmed, But New Questions. Oncology Times, 31. doi: 10.1097/01.COT.0000364239.69157.69. Accessed June 17, 2012.
National Cancer Institute. (n.d.). Tumor Markers. Accessed June 17, 2012.
National Comprehensive Cancer Network. (2012). NCCN Guidelines for Patients. Colon Cancer. Accessed June 26, 2012.