Screening can detect polyps before they become cancerous, as well as detecting colon cancer during its early stages when the chances of a cure are much higher. The following are the most common screening and diagnostic procedures for colorectal cancer:
Fecal occult blood test (blood stool test) – this checks a sample of the patient’s stool (feces) for the presence of blood. This can be done at the GP’s (general practitioner’s, primary care physician’s) office. However, most patients are given a kit that explains how to take the sample at home. The patient then returns the sample to the doctor’s office, and it is sent to a laboratory.
A blood stool test is not 100% accurate – it might not detect all cancers because not all of them bleed. Even cancers that do bleed often do not do so all the time. Therefore, it is possible that a patient has a negative result, even though he/she has cancer. Even if blood is detected, this may be caused by other illnesses or conditions, such as hemorrhoids. Some foods may suggest blood in the colon, when in fact, none was present.
Stool DNA test – this test analyzes several DNA markers that colon cancers or precancerous polyps cells shed into the stool. Patients may be given a kit with instructions on how to collect a stool sample at home. This has to be brought back to the doctor’s office, and is then sent to a laboratory.
This test is much more accurate for detecting colon cancer than polyps. However, it cannot detect all DNA mutations which may indicate that a tumor is present.
Flexible sigmoidoscopy – the doctor uses a sigmoidoscope, a flexible, slender and lighted tube, to examine the patient’s rectum and sigmoid (the sigmoid colon is the last of the colon, before the rectum). The test does not generally take more than a few minutes and is not painful; but might be uncomfortable. There is a small risk of perforation of the colon wall. If the doctor detects a polyps or colon cancer he/she will then carry on a colonoscopy to examine the entire colon and take out any polyps that are present – they will then be examined under a microscope.
A sigmoidoscopy will only detect polyps or cancer present at the end third of the colon and the rectum. If there are any in any other parts of the digestive tract it will not detect them.
Barium enema X-ray – Barium is a contrast dye that is placed into the patient’s bowel in an enema form – it shows up on an X-ray. In a double-contrast barium enema air is added as well. The barium fills and coats the lining of the bowel, creating a clear image of the rectum, colon, and occasionally of a small part of the patient’s small intestine. This procedure is often carried out along with a flexible sigmoidoscopy to detect any small polyps the barium enema X-ray may have missed. If the barium enema X-ray detects anything abnormal, the doctor may recommend a colonoscopy.
Colonoscopy – the doctor uses a colonoscope, which is much longer than a sigmoidoscope. A colonoscope is a long, flexible and slender tube which is attached to a video camera and monitor. The doctor can see the whole of the colon and rectum. Any polyps discovered during this exam can be removed there and then – sometimes tissue samples (biopsies) may be taken instead. Taking biopsies does not hurt.
Although colonoscopies are painless, some patients are given a mild sedative to calm them down. Prior to the exam the patient may be given a large amount of laxative fluid to clean out the colon (enemas are rarely used). Bleeding and perforation of the colon wall are possible complications, but extremely rare.
CT colonography (virtual colonoscopy) – a CT (computerized tomography) machine is used to take images of the colon. The patient needs to have a cleared colon for this exam to be effective. Even if anything abnormal is detected, the patient will then need conventional colonoscopy. A study found that CT colonography may offer patients at increased risk of colorectal cancer an alternative to colonoscopy that is less-invasive, is better-tolerated and has good diagnostic accuracy.
Ultrasound scan – sound waves are used to help show if the cancer has spread to another part of the body.
Magnetic resonance imaging (MRI) – this gives a three-dimensional image of the bowel and may help the doctor in his/her diagnosis.