Introduction: Who Should Get Tested and When

Regular screening for colon cancer is one of the most important steps you can take to protect your health. Medical experts recommend that most people start getting screened at age 45, even if they have no symptoms. This is because colon cancer often develops without warning signs in its early stages, and finding it early dramatically improves the chances of successful treatment.^[1]

You should consider getting tested sooner if you belong to certain higher-risk groups. If you are Black, you face a higher risk than people of other racial backgrounds. Women who are 50 years or older also have increased risk. Other factors that make earlier or more frequent screening important include having a family member who had colon or rectal cancer, inheriting certain gene mutations, having inflammatory bowel diseases like ulcerative colitis or Crohn disease, or having had colon polyps in the past.^[1][9]

Even if you’re not yet at the recommended screening age, you should see a doctor right away if you notice certain symptoms that won’t go away. These warning signs include blood in your stool or bleeding from your rectum, persistent changes in bowel habits such as ongoing constipation or diarrhea, unexplained belly pain, feeling full even after having a bowel movement, losing weight without trying, or feeling unusually tired or weak all the time.^[1][12]

It’s important to understand that many symptoms of colon cancer can also be caused by less serious conditions. Blood in your stool might come from hemorrhoids, belly pain could be related to digestive issues, and changes in bowel habits might result from dietary changes or stress. However, because these symptoms can also signal cancer, it’s always better to get checked by a healthcare professional rather than assuming the cause is harmless.^[12]

Diagnostic Methods for Identifying Colon Cancer

Screening Tests

Screening tests are designed to find colon cancer or precancerous polyps before they cause symptoms. The most comprehensive screening tool is a colonoscopy, which allows doctors to examine the entire length of your colon and rectum. During this procedure, a doctor inserts a thin, flexible tube with a camera on the end through your rectum. The camera sends images to a monitor, letting the doctor see the inside of your large intestine in real time. If the doctor spots any polyps or unusual areas during the colonoscopy, they can remove tissue samples or even take out small polyps right away.^[1][8]

A sigmoidoscopy is similar to a colonoscopy but examines only the lower part of the colon. While it doesn’t provide a view of the entire colon, it can still detect problems in the sigmoid colon and rectum. Both procedures require preparation beforehand to clean out your bowel, which usually involves following a special diet and taking laxatives the day before the test.^[1]

Another screening option is the fecal occult blood test, which looks for hidden blood in your stool that you might not be able to see with your eyes. This is a simpler test that you can often do at home, but if it shows positive results, you will need a colonoscopy to investigate further. Some colon cancers bleed in amounts too small to notice, but this test can pick up those tiny traces.^[1]

Biopsy

When doctors find a suspicious area during a colonoscopy or other examination, they need to take a small piece of tissue to examine under a microscope. This procedure is called a biopsy. The tissue sample is sent to a laboratory where specialists called pathologists look at the cells to determine if they are cancerous. The biopsy is the only way to definitively confirm that someone has colon cancer and to identify what type of cancer it is.^[1][8]

Most colon cancers are adenocarcinomas, which means they start in the glands that line the inside of the colon and produce mucus. These glands normally help keep the colon lubricated and functioning properly. When the cells in these glands begin to change and grow out of control, they can form tumors. Over 95% of colorectal cancers fall into this category, which is why when doctors talk about colon cancer, they are usually referring to adenocarcinoma.^[5][14]

Blood Tests

While blood tests alone cannot diagnose colon cancer, they provide valuable information about your overall health and can help doctors plan your care. A complete blood count can reveal if you have a low number of red blood cells, a condition called anemia, which might indicate that a tumor is bleeding inside your colon. Other blood tests check how well your liver and kidneys are working, which becomes especially important when planning treatment.^[1][8]

Doctors may also test your blood for a protein called carcinoembryonic antigen, or CEA. Some colon cancers produce this protein, and measuring its level can help track how well treatment is working or whether cancer has come back after treatment. However, not all colon cancers make CEA, and some people without cancer can have elevated levels, so this test is used alongside other diagnostic methods rather than on its own.^[8]

Imaging Tests

Once a biopsy confirms colon cancer, doctors use various imaging tests to see how far the cancer has spread. These tests create detailed pictures of the inside of your body without requiring surgery. A computed tomography scan, or CT scan, uses X-rays taken from different angles to create cross-sectional images of your abdomen, pelvis, and chest. This helps doctors see if the cancer has spread to nearby lymph nodes or to distant organs like the liver or lungs.^[1][8]

Magnetic resonance imaging, or MRI, uses powerful magnets and radio waves instead of X-rays to create detailed images of soft tissues. This test is particularly useful for examining the liver and brain. Some people find MRI scans challenging because they must lie still inside a narrow tube for an extended period, but the test is painless and doesn’t expose you to radiation.^[1]

Ultrasound uses sound waves to create images of your internal organs. Doctors sometimes perform a special type of ultrasound called transrectal ultrasound, where they insert a small probe into the rectum to get a closer look at the rectal wall and nearby structures. This helps determine how deeply a tumor has grown into the intestinal wall.^[1]

Positron emission tomography, or PET scan, involves injecting a small amount of radioactive sugar into your bloodstream. Cancer cells tend to use more sugar than normal cells, so they show up as bright spots on the scan. PET scans are sometimes combined with CT scans to provide both metabolic and structural information about potential cancer sites.^[1]

Traditional X-rays may also be used, sometimes with a substance called barium. In a barium enema, liquid barium is placed into your rectum before X-rays are taken. The barium coats the lining of your colon and shows up white on the X-rays, making it easier to see the shape and structure of your intestines and spot any abnormal areas.^[1]

Diagnostics for Clinical Trial Qualification

When patients consider joining clinical trials to test new treatments for colon cancer, they must undergo specific diagnostic tests to determine if they qualify for the study. These qualification criteria help researchers ensure that the treatments being tested are evaluated in the right patient populations and that the results will be meaningful.

Most clinical trials require confirmation of colon cancer through a biopsy before enrollment. Researchers need to verify not only that cancer is present but also what type it is and certain characteristics of the cancer cells. One increasingly important factor is whether the tumor has high microsatellite instability, abbreviated as MSI-H, or DNA mismatch repair deficiency, known as dMMR. These are genetic features that affect how the cancer behaves and responds to treatment. About 5% of colon cancers have these characteristics, and they often respond differently to certain immunotherapy treatments.^[9][14]

Genetic testing of tumor tissue has become a standard part of qualifying for many clinical trials. Researchers look for mutations in specific genes such as EGFR, KRAS, and BRAF. These mutations influence how cancer cells grow and spread, and they determine which targeted therapies might work. For example, patients whose tumors have certain KRAS mutations would not benefit from some types of targeted therapy, so clinical trials testing those therapies would exclude these patients.^[9][14]

Another measurement used in clinical trial qualification is tumor mutational burden, or TMB. This refers to the total number of mutations present in the cancer cells. Tumors with high tumor mutational burden, designated TMB-H, may respond better to certain immunotherapies because they produce more abnormal proteins that the immune system can recognize and attack.^[14]

Blood tests play an important role in determining clinical trial eligibility. Most trials require baseline blood work to ensure your liver, kidneys, and bone marrow are functioning well enough to handle the experimental treatment. Your blood cell counts, including red blood cells, white blood cells, and platelets, must typically fall within certain ranges. These requirements protect patient safety by ensuring that participants can withstand the potential side effects of the treatment being tested.^[1]

Imaging studies are also essential for clinical trial qualification. Researchers need baseline CT scans, MRI scans, or PET scans to measure the size and location of tumors before treatment begins. These initial images serve as comparison points for later scans that will show whether the experimental treatment is working. Trials may have specific criteria about how large tumors must be or whether cancer has spread to certain organs.^[1]

Performance status is another factor evaluated for clinical trial enrollment. Doctors use standardized scales to assess how well you can perform everyday activities. This helps determine whether you are physically strong enough to participate in the study. Most trials require that patients are able to care for themselves and spend most of their waking hours out of bed, even if they cannot work full-time.

Some trials specifically recruit patients whose cancer has spread to distant organs, a condition called metastatic cancer, while others focus on earlier-stage disease. Your diagnostic imaging results determine which category you fall into. Cancer that has spread beyond the colon to the liver, lungs, or other organs is classified as stage IV disease, and many clinical trials are designed specifically for patients at this stage who have already tried standard treatments.^[9]

Prior treatment history affects clinical trial eligibility. Some trials are designed for patients who have never received treatment, while others specifically enroll patients whose cancer has returned or progressed after previous therapy. You’ll need detailed records of any chemotherapy, radiation, surgery, or other treatments you’ve received, including when you received them and how well they worked.

Age can also be a factor in clinical trial qualification, though many trials now include participants across a wide age range. Some studies focus specifically on younger adults or older individuals to understand how treatments work in these populations. Your diagnostic tests remain the same regardless of the trial’s age criteria.^[9]