Colon cancer is cancer that originates in the large intestine, or the colon; and rectal starts in the end of the large intestine. The term colorectal cancer can describe either of these conditions. Colorectal cancer typically does not cause any symptoms in its early stages, but once symptoms appear, further tests need to be done to definitively diagnose the disease.
Physical Exams
When colorectal cancer is suspected, along with a physical exam, a digital rectal exam (DRE) may be performed. The National Cancer Institute describes the DRE as involving a health care provider inserting a lubricated, gloved finger into the rectum to feel for any abnormalities in the lower area of the rectum. A fecal occult blood test will also be done. This test checks stool for microscopic traces of blood, which can indicate colorectal cancer. Stool samples are taken and smeared on a special card, which is then tested for blood.
Blood Tests
Some blood tests are used in colorectal cancer, such as a complete blood count (CBC) and tumor markers. A CBC can help determine whether anemia, or decreased red blood cell count, is present. Colorectal cancer can sometimes cause bleeding, leading to weakness and anemia. A liver function test might be ordered with the CBC, as this is a common organ that colorectal cancer spreads to. Tumor marker blood tests are typically used to monitor colorectal cancer's response to treatment or to check for possible recurrences. These tests are not used for diagnosis, because these markers may also be elevated in individuals without cancer, according to the American Cancer Society. Colorectal cancer cells can produce carcinoembryonic antigen (CEA) and CA 19-9, and these substances are what the tumor marker tests help identify.
Pathologic Tests
If more information is needed after a physical exam and blood work, tests specifically looking for polyps or cancer may be done. A biopsy takes a piece of tissue from a suspicious area from the colon or rectum, typically during a colonoscopy. This tissue sample is sent to a lab, where a pathologist examines the cells under a microscope to check for cancer. Tests may also be done to look for a gene called K-ras, whose mutations have been found in 4 out of 10 colorectal cancers, according to the American Cancer Society. Changes in a gene called BRAF may also be seen; these genetic mutations are important because they can help guide treatment options, as these changes are not responsive to all treatments.
Imaging Tests
Tests that use X-rays, sound waves or radioactive substances to help produce images on the inside of the body are called imaging tests. These tests can help diagnose cancer, assess the spread and extent of cancer and monitor the cancer's response to treatment. Tests in this category include a CT scan, MRI, ultrasound, chest X-ray, PET scan and a CT-guided needle biopsy.
