How is colorectal cancer diagnosed?

Colorectal cancer is often found after symptoms appear, but most people with early colon or rectal cancer don’t have symptoms of the disease. Symptoms usually only appear with more advanced disease. This is why getting the recommended screening tests (described in the section “Can colorectal polyps and cancer be found early?”) before any symptoms develop is so important.

If your doctor finds something suspicious during a screening exam, or if you have any of the symptoms of colorectal cancer described in the section “Signs and symptoms of colorectal cancer,” your doctor will probably recommend exams and tests to find the cause.

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 might also order certain blood tests to help determine if you have colorectal cancer or to help monitor your disease if you’ve been diagnosed with 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.

Liver enzymes: You may also have a blood test to check your liver function, because colorectal cancer can spread to the liver.

Tumor markers: Colorectal cancer cells sometimes make substances, like carcinoembryonic antigen (CEA) and CA 19-9, that are released into the bloodstream. Blood tests for these tumor markers are used most often along with other tests to monitor patients who already have been diagnosed with or treated for colorectal cancer. They may help show how well treatment is working or provide an early warning of a cancer that has returned.

These tumor markers are not used to screen for or diagnose colorectal cancer because the tests can’t tell for sure whether or not someone has cancer. Tumor marker levels can sometimes be normal in a person who has cancer and can be abnormal for reasons other than cancer. For example, higher levels may be found 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 more tests. This most often is colonoscopy, but sometimes a sigmoidoscopy or an imaging test such as a barium enema (lower GI series) may be done first. These tests are described in detail in the section “Can colorectal polyps and cancer be found early?”


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. There may be some bleeding afterward, but this usually stops after a short time. Less often, part of the colon may need to be surgically removed to make the diagnosis. See Testing Biopsy and Cytology Specimens for Cancer to learn more about the types of biopsies, how the tissue is used in the lab to diagnose cancer, and what the results may show.

Lab tests of samples

Biopsy samples (from colonoscopy or surgery) are sent to the lab where a pathologist, a doctor trained to diagnose cancer and other diseases in tissue samples, looks at them under a microscope. Other tests may suggest that colorectal cancer is present, but the only way to be sure is to look at the samples under a microscope.

Gene tests: Other lab tests may also be done on biopsy specimens to help better classify the cancer. Doctors may look for specific gene changes in the cancer cells that might affect how the cancer is best treated. For example, doctors now typically test the cells for changes in the KRAS gene. This gene is mutated in about 4 out of 10 colorectal cancers. Some doctors may also test for changes in the BRAF gene. Patients with cancers with mutations in either of these genes do not benefit from treatment with certain anti-cancer drugs such as cetuximab (Erbitux®) and panitumumab (Vectibix®).

MSI testing: Sometimes the tumor tissue will be tested to see if it shows changes called microsatellite instability (MSI). This change is present in most colorectal cancers caused by hereditary non-polyposis colon cancer (HNPCC) and can also affect some cancers in patients who do not have HNPCC. There are 2 reasons to test colorectal cancers for MSI. The first reason is to identify patients who may have HNPCC. If they are found to have MSI, they can be tested for HNPCC. A diagnosis of HNPCC helps plan further screening for the patient (for example women with HNPCC may need to be screened for uterine cancer. Also, if the patient is known to have HNPCC, their relatives could also have it, and may want to be tested for it. If they do have HNPCC, they are at increased risk of developing cancer and would need to be screened accordingly. The second reason is that knowing an early-stage colorectal cancer has MSI may change the way it is treated.

Some doctors suggest MSI testing only if a patient meets Bethesda criteria. Others test all colorectal cancers for MSI, and still others decide based on the age of the patient or the stage of the cancer. There are several ways to test for MSI. One way is to start with a DNA test for MSI. Another way is to first do an immunohistochemistry test to see if certain proteins related to MSI are missing in the cancer cells. If that test looks suspicious, then the DNA test for MSI is done. Not all patients whose cancer cells show MSI have HNPCC. Patients who want to be tested for HNPCC will have a test of the DNA in their blood cells to look the genetic changes that cause HNPCC.

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.

Before the scan, you may be asked to drink a contrast solution and/or get an intravenous (IV) injection of a contrast dye that helps better outline abnormal areas in the body. You may need an IV line through which the contrast dye is injected. The injection can cause some flushing (redness and warm feeling). Some people are allergic and get hives or, rarely, more serious reactions like trouble breathing and low blood pressure. Be sure to tell the doctor if you have any allergies or if you 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 slides in and out of a ring-shaped scanner. You might feel a bit confined by the ring while the pictures are being taken.

CT with portography looks specifically at the portal vein, the large vein leading into the liver from the intestine. In this test, contrast material is injected into veins that lead to the liver, to look better at colorectal cancer that has spread to the liver.

CT-guided needle biopsy: In cases where a suspected area of cancer lies deep within the body, a CT scan can be used to guide a biopsy needle precisely into the suspected area. For this procedure, the patient remains on the CT scanning table, while the doctor advances a biopsy needle through the skin and toward the mass. CT scans are repeated until the doctor can see 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) is then removed and looked at under a microscope. This is not used to biopsy a colon tumor, but is often done if the CT shows tumors in the liver.


Ultrasound uses 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. For the exam, you simply lie on a table and a technician moves the transducer along the skin overlying the part of your body being examined. Usually, the skin is first lubricated with gel.

Abdominal ultrasound can be used to look for tumors in your liver, gallbladder, pancreas, or elsewhere in your abdomen, but it can’t look for tumors of the colon. Two special types of ultrasound exams are sometimes used to evaluate colon and rectal cancers.

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

Intraoperative ultrasound: This exam is done during surgery after the surgeon has opened the abdominal cavity. The transducer can be placed against the surface of the liver, making this test very useful for detecting the spread 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 by the body 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 may be 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, more open MRI machines can sometimes help with this if needed, but the images may not be as sharp in some cases. MRI machines make buzzing and clicking noises that you may find disturbing. Some centers provide earplugs to help block this noise out.

MRI scans can be helpful in patients with rectal cancers to see if the tumor has spread into nearby structures. 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 during the test and can be uncomfortable.

MRI is also sometimes useful in looking at abnormal areas in the liver that might be due to cancer spread or to look at the brain and spinal cord.

Chest x-ray

This test may be done after colorectal cancer has been diagnosed to see if cancer has spread to the lungs.

Positron emission tomography (PET) scan

For a PET scan, a form of radioactive sugar (known as fluorodeoxyglucose or FDG) is injected into the blood. The amount of radioactivity used is very low. Cancer cells in the body grow rapidly, so they absorb large amounts of the radioactive sugar. After about an hour, you will be moved onto a table in the PET scanner. You lie on the table for about 30 minutes while a special camera creates 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.

A PET scan can help give the doctor a better idea of whether an abnormal area seen on another imaging test is a tumor or not. If you have already been diagnosed with cancer, your doctor may use this test to see if the cancer has spread to lymph nodes or other parts of the body. A PET scan can also be useful if your doctor thinks the cancer may have spread but doesn’t know where.

Special machines are able to perform both a PET and CT scan at the same time (PET/CT scan). This allows the doctor to compare areas of higher radioactivity on the PET with the more detailed picture of that area on the CT.


Angiography is an x-ray procedure for looking at blood vessels. Contrast medium, or dye, is injected into an artery before x-ray images are taken. The dye outlines the blood vessels on x-ray pictures.

If your cancer has spread to the liver, angiography can be useful in showing the arteries that supply blood to those tumors. This can help surgeons decide if the liver tumors can be removed and if so, it can help in planning the operation. Angiography can also be helpful in planning other treatments for cancer spread to the liver, like embolization (this is discussed in the section about surgery).

Angiography can be uncomfortable because the doctor who does the procedure has to put a small catheter (a flexible hollow tube) into the artery leading to the liver to inject the dye. Usually the catheter is put into an artery in your inner thigh and threaded up into the liver artery. You have to hold very still while the catheter is in place. A local anesthetic is often used to numb the area before inserting the catheter. Then the dye is injected quickly to outline all the vessels while the x-rays are being taken.

Angiography may also be done with a CT scanner (CT angiography) or an MRI scanner (MR angiography). These techniques give information about the blood vessels in the liver without the need for a catheter in the leg artery, although you may still need an IV line so that a contrast dye can be injected into the bloodstream during the imaging.