If your doctor finds something suspicious during a screening exam, or if you have any of the symptoms of colorectal cancer described below, you will likely need to undergo a diagnostic workup.

Signs and symptoms of colorectal cancer

If you have any of the following you should check with your doctor for prompt diagnosis and treatment:

• a change in bowel habits, such as diarrhea, constipation, or narrowing of the stool, that lasts for more than a few days 
• a feeling that you need to have a bowel movement that is not relieved by doing so
• rectal bleeding, dark stools, or blood in the stool (often, though, the stool will look normal) 
• cramping or abdominal (stomach area) pain 
• weakness and fatigue

Most of these symptoms are more likely to be caused by conditions other than colorectal cancer, such as infection, hemorrhoids, or inflammatory bowel disease. Still, if you have any of these problems, it's important to see your doctor right away so the cause can be found and treated, if needed.

Whether you are undergoing diagnosis because of the results of a screening exam or because you have symptoms, your doctor may perform the following:

Medical history and physical exam

If you have any signs or symptoms that suggest you might have colorectal cancer, your doctor will want to take a complete medical history to check for symptoms and risk factors, including your family history.

As part of a physical exam, your doctor will carefully feel your abdomen for masses or enlarged organs, and also examine the rest of your body. Your doctor may also perform a digital rectal exam (DRE). During this test, the doctor inserts a lubricated, gloved finger into the rectum to feel for any abnormal areas.

Blood tests

Your doctor may also order certain blood tests to help determine if you have colorectal cancer.

Complete blood count (CBC): Your doctor may order a complete blood count to see if you have anemia (too few red blood cells). Some people with colorectal cancer become anemic because of prolonged bleeding from the tumor. You may also have a blood test of your liver function, because colorectal cancer can spread to the liver and cause abnormalities.

Tumor markers: Colorectal cancer sometimes produces substances, such as carcinoembryonic antigen (CEA) and CA 19-9, that are released into the bloodstream. Blood tests for these "tumor markers" are used most often with other tests for follow-up of patients who already have been treated for colorectal cancer. They may provide an early warning of a cancer that has returned.

These tumor markers are not used to find cancer in people who have never had a cancer and appear to be healthy because the tests are not always accurate. Tumor marker levels can be normal in a person who has cancer and can be abnormal for reasons other than cancer. For example, higher levels may also be present in the blood of some people with ulcerative colitis, non-cancerous tumors of the intestines, or some types of liver disease or chronic lung disease. Smoking can also raise CEA levels.

Tests to look for colorectal polyps or cancer

If symptoms or the results of the physical exam or blood tests suggest that colorectal cancer might be present, your doctor may recommend one or more additional tests. This might include endoscopic tests such as sigmoidoscopy or colonoscopy, or imaging tests such as a barium enema (lower GI series), double-contrast barium enema, or CT colonography (virtual colonoscopy). Most of these tests are described in detail in the section "Can colorectal polyps or cancer be found early?"

Biopsy: Usually if a suspected colorectal cancer is found by any diagnostic test, it is biopsied during a colonoscopy. In a biopsy, the doctor removes a small piece of tissue with a special instrument passed through the scope. Although there may be some bleeding afterward, this usually stops after a short time.

The biopsy specimen is sent to the lab where a pathologist, a doctor trained to diagnose cancer and other diseases in tissue samples, looks at the tissue under a microscope. While other tests may suggest that colorectal cancer is present, a biopsy is the only way to determine this for certain.

Imaging tests

Imaging tests use sound waves, x-rays, magnetic fields, or radioactive substances to create pictures of the inside of your body. Imaging tests may be done for a number of reasons, including to help find out whether a suspicious area might be cancerous, to learn how far cancer may have spread, and to help determine if treatment has been effective.

Computed tomography (CT or CAT) scan

The CT scan is an x-ray test that produces detailed cross-sectional images of your body. Instead of taking one picture, like a regular x-ray, a CT scanner takes many pictures as it rotates around you while you lie on a table. A computer then combines these pictures into images of slices of the part of your body being studied. Unlike a regular x-ray, a CT scan creates detailed images of the soft tissues in the body. This test can help tell if colon cancer has spread into your liver or other organs.

After the first set of pictures is taken you may be asked to drink a contrast solution and/or receive an IV (intravenous) line through which a contrast dye is injected. This helps better outline structures in your body. A second set of pictures is then taken.

The contrast may cause some flushing (a feeling of warmth, especially in the face). Some people are allergic and get hives. Rarely, more serious reactions like trouble breathing or low blood pressure can occur. Be sure to tell the doctor if you have ever had a reaction to any contrast material used for x-rays.

CT scans take longer than regular x-rays. You need to lie still on a table while they are being done. During the test, the table moves in and out of the scanner, a ring-shaped machine that completely surrounds the table. You might feel a bit confined by the ring you have to lie in while the pictures are being taken.

In recent years, spiral CT (also known as helical CT) has become available in many medical centers. This type of CT scan uses a faster machine. The scanner part of the machine rotates around the body continuously, allowing doctors to collect the images much more quickly than with standard CT. This lowers the chance of "blurred" images occurring as a result of breathing motion. It also lowers the dose of radiation received during the test. The biggest advantage may be that the "slices" it images are thinner, which yields more detailed pictures and allows doctors to look at suspicious areas from different angles.

For spiral CT with portography (looking at the portal vein -- the large vein leading into the liver from the intestine), contrast material is injected into veins that lead to the liver, to help find metastases from colorectal cancer to that organ.

CT-guided needle biopsy: CT scans can also be used to precisely guide a biopsy needle into a suspected tumor or metastasis. For this procedure, the patient remains on the CT scanning table, while a radiologist advances a biopsy needle through the skin and toward the location of the mass. CT scans are repeated until the doctors are confident that the needle is within the mass. A fine-needle biopsy sample (tiny fragment of tissue) or a core needle biopsy sample (a thin cylinder of tissue about ½ inch long and less than 1/8 inch in diameter) is then removed and looked at under a microscope.

CT colonography (virtual colonoscopy): CT scans can also be used to perform a "virtual colonoscopy." This test requires the same type of preparation (cleansing of stool from the colon) as is needed before colonoscopy. Before the scan is done, the colon is inflated with air so that it can be viewed more clearly; this stretches the colon and can cause some discomfort.

Spiral CT of the abdomen is then done. The thin images it obtains can be combined to create two- and three-dimensional views of the colon and rectum. If abnormalities are detected, a follow-up colonoscopy will be needed to take tissue samples of the abnormal areas.

Ultrasound

Ultrasound involves the use of sound waves and their echoes to produce a picture of internal organs or masses. A small microphone-like instrument called a transducer emits sound waves and picks up the echoes as they bounce off body tissues. The echoes are converted by a computer into a black and white image that is displayed on a computer screen. This test is painless and does not expose you to radiation.

Abdominal ultrasound can be used to look for tumors in your liver, gallbladder, pancreas, or even inside your abdomen, although it can't look for tumors of the colon.

When you have an abdominal ultrasound exam, you simply lie on a table and a technician moves the transducer over the skin overlying the part of your body being examined. Usually, the skin is first lubricated with gel.

Two special types of ultrasound exams can be used to evaluate people with colon and rectal cancer.

Endorectal ultrasound uses a special transducer that can be inserted directly into the rectum. This test is used to see how far through the wall a rectal cancer may have penetrated and whether it has spread to nearby organs or tissues such as lymph nodes.

Intraoperative ultrasound is done after the surgeon has opened the abdominal cavity. The transducer can be placed against the surface of the liver, making this test very useful in detecting metastases of colorectal cancer to the liver.

Magnetic resonance imaging (MRI) scan

Like CT scans, MRI scans provide detailed images of soft tissues in the body. But MRI scans use radio waves and strong magnets instead of x-rays. The energy from the radio waves is absorbed and then released in a pattern formed by the type of body tissue and by certain diseases. A computer translates the pattern into a very detailed image of parts of the body. A contrast material called gadolinium is often injected into a vein before the scan to better see details.

MRI scans are a little more uncomfortable than CT scans. First, they take longer -- often up to an hour. Second, you have to lie inside a narrow tube, which is confining and can upset people with claustrophobia (a fear of enclosed spaces). Newer, "open" MRI machines can sometimes help with this if needed. The machine also makes buzzing and clicking noises that you may find disturbing. Some centers provide headphones with music to block this out.

MRI scans are sometimes useful in looking at abnormal areas in the liver that might be due to cancer spread. They can also help determine the extent of rectal cancers. To improve the accuracy of the test, some doctors use endorectal MRI. For this test the doctor places a probe, called an endorectal coil, inside the rectum. This must stay in place for 30 to 45 minutes and can be uncomfortable.

Chest X-ray

This test may be done after colorectal cancer has been diagnosed to determine whether it has spread to the lungs.

Positron emission tomography (PET) scan

PET scans involve injecting a form of radioactive sugar (known as fluorodeoxyglucose or FDG) into the blood. The amount of radioactivity used is very low. Because cancer cells in the body are growing rapidly, they absorb large amounts of the radioactive sugar. A special camera can then create a picture of areas of radioactivity in the body. The picture is not finely detailed like a CT or MRI scan, but it provides helpful information about your whole body.

PET scans are sometimes useful if your doctor thinks the cancer may have spread (or returned after treatment) but doesn't know where. PET scans can be used instead of several different x-rays because they scan your whole body.

Some newer machines are able to perform both a PET and CT scan at the same time (PET/CT scan). This allows the radiologist to compare areas of higher radioactivity on the PET with the appearance of that area on the CT.

Angiography

This test is sometimes used to help plan surgery, especially for tumors in the liver. For an angiogram, a doctor inserts a very thin tube (called a catheter) into an artery, usually on the inner thigh. The catheter is threaded through the artery until the tip is near the liver. Contrast dye is then injected rapidly and a series of x-rays is taken. This can show surgeons the location of blood vessels next to any tumors in the liver, so that they can be removed without causing a lot of bleeding.