Can Cancer Be Detected by a Blood Test? Colon Cancer Screening Tests

What types of blood tests can help detect cancer?

Tumor marker tests

Tumor markers are chemicals produced by cancer cells and found in the blood. However, tumor markers are also made by normal cells in the body, and levels may increase significantly in noncancerous conditions. Examples of tumor markers include:

Circulating tumor cell tests

Recently developed blood tests can detect tumor cells that have separated from an original cancer site and are flowing through the bloodstream. The United States Food and Drug Administration (U.S. FDA) has approved one circulating tumor cell test to monitor people with breast, colorectal or prostate cancer. However, these tests are not commonly used in a clinical setting.

Blood protein testing

Electrophoresis is a test that examines various proteins in the blood. This can help identify various immunoglobulins, which are elevated in people with multiple myeloma. A bone marrow biopsy may help confirm the diagnosis of blood cancer.

Complete blood count (CBC)

A complete blood count helps the doctor to:

• Diagnose some blood cancers, such as leukemia and lymphoma.
• Find out if cancer has invaded the bone marrow.
• Look how a person’s body handles cancer treatment.
• Diagnose other noncancerous conditions.

The results of CBC are interpreted as:

• Low white blood cell count: Cancer treatment tends to lower the body’s white blood cells. Leukemia, lymphoma or multiple myeloma also lower white blood cell count.
• Low red blood cell count: Some chemotherapy or radiation treatment may lower red blood cell count (anemia). Low red blood cell count could also be caused by the cancer bleeding into the stomach, which occurs in stomach cancer.
• Varying amount of white blood cells: Highly abnormal lymphocytes or monocytes can indicate the possibility of certain types of cancer.
• Low platelet count: Cancer that affects the bone marrow can also lower platelet count.

A bone marrow biopsy may help confirm the diagnosis of blood cancer.

Urinalysis

Urinalysis is a lab examination of urine for various cells and chemicals, such as red blood cells, white blood cells, infection or excessive protein. Blood in the urine may indicate a benign condition, but it can also indicate other problems.

Other blood tests include:

• Alkaline phosphatase: It helps to diagnose liver cancer and bone cancer.
• Ferritin: Extremely elevated levels of ferritin may indicate Hodgkin disease or leukemia.
• Thyroglobulin: Elevated levels of this hormone may indicate follicular cancer.

Is there a blood test for colon cancer?

Your doctor may order blood tests to look for signs of colon cancer

No blood test can detect colon cancer. However, your doctor may order blood tests to get a picture of your overall health, as well as use other screening methods to look for signs of colon cancer.

Your doctor may want to test your blood for a substance called carcinoembryonic antigen (CEA), which is produced by some colon cancer cells. Tracked over time, the level of CEA in the blood may help your doctor understand your treatment outlook and whether the cancer is responding to the treatment protocol. However, this test is not specific to colon cancer and may also be negative in other types of colon cancers.

It should be noted that research is being conducted in Taiwan for a blood test that can detect colon cancer. The test detects circulating tumor cells (CTCs) in the blood. According to the study, the blood test identified colon cancer in 87% of cases, ranging from stage I to stage IV cancer. The blood test was also able to detect 77%  of precancerous lesions, indicating an early-stage disease. 

While these early results appear promising, the sensitivity of the test is still not optimal. The number of patients in the study is also relatively small. Therefore, the test is not available in the U.S. yet.

What types of screening methods are available for colon cancer?

Fecal occult blood test (FOBT)

Your doctor may recommend a fecal occult blood test (FOBT) to screen for colon cancer. FOBT is a test that checks the stool (solid waste) for blood, using a microscope. A small sample of stool is placed on a special card or in a special container and returned to the doctor or laboratory for testing. 

While blood in the stool can be an early sign of colon cancer, it may also be a sign of other conditions, such as infections, polyps, injuries or other noncancerous diseases. There are two types of FOBTs:

• Guaiac FOBT: The sample of stool is placed on the special card and then tested with a chemical. If there is blood in the stool, the special card changes its color.
• Immunochemical FOBT: A liquid is added to the stool sample. This mixture is injected into a machine that contains antibodies that can detect blood in the stool. If there is blood in the stool, a line appears on the window of the machine. This test is also called the fecal immunochemical test (FIT).

Cologuard

Cologuard is a new screening method that combines a FIT test with a deoxyribonucleic acid (DNA) test to detect existing colon cancer or advanced precancerous polyps. Cells from the lining of the large intestine are shed periodically and are passed through the stool. If there are cancerous polyps in the large intestine, cells from these polyps are also shed and can be detected in the stool. Cologuard looks for DNA associated with colon cancer in these shed cells as well as the presence of blood in the stool.

Colonoscopy

• During a colonoscopy, the gastroenterologist (doctor specializing in intestines) observes the inside of the large intestine (colon) for specific signs of colon cancer such as polyps (abnormal growths that could turn into cancer).
• The doctor usually inserts a thin, flexible tube with a camera and a light attached on the end (called a colonoscope) into the rectum and colon.
• If the doctor finds polyps, they may remove them and send them to a lab for further testing.
• The results from these tests will determine whether the polyp is benign or cancerous.

Computed tomography (CT) colonography

• CT colonography or virtual colonography uses CT technology to produce multiple cross-sectional images of the intestinal tract.
• These images are combined on a computer to produce detailed images of the entire length of the colon. The doctor uses these images to identify polyps or abnormal tissue that could be precancerous or cancerous.
• To capture small defects in the wall, the colon is slightly inflated with air. This is done by inserting a small tube slightly into the rectum.
• This exam is strictly diagnostic and not therapeutic. This means that the CT can find lesions, such as polyps or cancers, but cannot remove them. Any abnormal appearing lesions will require a colonoscopy exam for the removal or tissue sampling.

Flexible sigmoidoscopy

• Flexible sigmoidoscopy exams are essentially limited colonoscopies.
• The same equipment used in colonoscopy is used for this, but only the left side of the colon is examined.
• This exam is less invasive than a colonoscopy, carries less risk and usually, can be done without sedation due to minimal discomfort.
• Just like a colonoscopy, polyp removal or biopsies can be done during the exam.
• Since this exam only looks at the more distal colon lesions, the rest of the colon is not examined.

Barium enema

• A series of X-rays of the lower gastrointestinal tract is usually evaluated during this procedure.
• A liquid that contains barium (a silver-white metallic compound) is put into the rectum. The barium coats the lower gastrointestinal tract and X-rays are taken. This procedure is also called the lower gastrointestinal series.