Introduction: Who Should Seek Diagnostic Testing

Colorectal cancer is a disease where cells in the colon or rectum grow out of control. The colon is the large intestine, which absorbs water and nutrients from food, while the rectum is the passageway connecting the colon to the anus. Together, these parts play a crucial role in your digestive system^[1].

If you are 45 years old or older, you should talk to your doctor about getting screened for colorectal cancer. This is true even if you feel perfectly healthy and have no symptoms. Screening is recommended for adults at average risk starting at age 45, because catching problems early can prevent cancer from developing or make it much easier to treat^[1][8].

You may need to start screening earlier or get tested more frequently if you have certain risk factors. These include having a parent, sibling, or child who has had colorectal cancer, which increases your risk two to three times compared to someone without a family history. If a relative was diagnosed at a young age or if more than one family member has been affected, your risk can be three to six times higher^[7].

People with certain inherited conditions also face higher risks. About 5% of colorectal cancer cases are linked to genetic conditions like Lynch syndrome (also called hereditary non-polyposis colorectal cancer), familial adenomatous polyposis, or MUTYH-associated polyposis. These conditions are passed down through families and significantly increase cancer risk^[7][8].

Other factors that increase risk include having a personal history of colorectal cancer, ovarian cancer, or high-risk adenomas. High-risk adenomas are colorectal polyps that are 1 centimeter or larger, or that look abnormal under a microscope. Having chronic ulcerative colitis or Crohn’s disease for 8 years or more also raises your risk^[8][12].

You should seek diagnostic testing promptly if you notice any symptoms that might suggest colorectal cancer, even if you’re younger than 45. Warning signs include blood in or on your stool, which may appear bright red, dark, or cause your stool to look black and tarry. Persistent changes in bowel habits lasting more than a few days—such as diarrhea, constipation, or feeling like your bowel doesn’t empty completely—are also important to report to your doctor^[5][6].

Other symptoms that warrant medical attention include belly pain with no known cause that doesn’t go away or hurts significantly, unexplained weight loss, constant tiredness, bloating that lasts more than a week or gets worse, or stools that are narrower than usual. Some people also experience frequent gas pains, fullness, or cramping^[3][6][8].

Colorectal cancer is now increasingly affecting younger adults. Cases among people under 50 have been rising, and about 30% of diagnoses now occur in people under 55. Each day, approximately 56 new cases are diagnosed in patients aged 46 and younger. This is why the recommended screening age was lowered from 50 to 45 in recent years^[7].

Black individuals face higher rates of colorectal cancer and death from the disease compared to other racial groups. Men are also slightly more likely to develop colorectal cancer than women. Understanding your personal risk factors and discussing them openly with your doctor helps ensure you get the right screening at the right time^[6][8].

Classic Diagnostic Methods

Several diagnostic methods are used to identify colorectal cancer and distinguish it from other conditions. These tests range from simple visual examinations to more complex procedures that allow doctors to see inside your colon and rectum.

Physical and Digital Rectal Examination

Your doctor may begin with a physical exam, which includes a digital rectal exam. During this exam, your healthcare provider inserts a gloved, lubricated finger into your rectum to feel for abnormal growths or areas. While this exam cannot detect all colorectal cancers, especially those higher up in the colon, it can help identify problems in the rectum and lower part of the colon^[8].

Colonoscopy

A colonoscopy is one of the most comprehensive and commonly used tests for detecting colorectal cancer. During this procedure, a doctor inserts a long, flexible, slender tube called a colonoscope into your rectum. The tube has a tiny video camera and light at the end, which allows the doctor to view the entire colon and rectum on a monitor^[10].

Colonoscopy is powerful because it serves multiple purposes. If the doctor finds polyps during the exam, they can remove them right away using surgical tools passed through the colonoscope. The doctor can also take tissue samples, called biopsies, for laboratory testing. This makes colonoscopy both a screening test and a treatment, as removing polyps before they become cancerous can actually prevent cancer from developing^[1][10].

For people at average risk with normal results, colonoscopy is typically done every three to five years. If you had surgery for colon cancer, a colonoscopy is usually recommended one year after surgery to check for any new problems^[10].

Sigmoidoscopy

Sigmoidoscopy is similar to colonoscopy but examines only the lower part of the colon and rectum. The procedure uses a shorter, flexible tube with a camera. While it cannot view the entire colon like a colonoscopy can, sigmoidoscopy is useful for checking the sigmoid colon and rectum for polyps or other abnormalities^[8].

Proctoscopy

For people who had surgery through the anus to remove rectal cancer, proctoscopy may be recommended. This exam focuses specifically on the rectum and is typically done every 3 to 6 months during the first two years after surgery to monitor for any signs of cancer returning^[10].

Fecal Tests

Several stool-based tests can detect signs of colorectal cancer. The fecal occult blood test checks for hidden blood in your stool that you cannot see with your eyes. Blood in the stool can be a sign of polyps or cancer. The fecal immunochemical test, often abbreviated as FIT, is a more specific type of stool test that also looks for blood^[8].

These tests are non-invasive and can be done at home. You collect a stool sample and send it to a laboratory for analysis. If the test finds blood or other concerning signs, your doctor will recommend follow-up tests, usually a colonoscopy, to investigate further.

Biopsy

A biopsy involves removing a small sample of tissue for examination in a laboratory. For colorectal cancer, tissue samples are often collected during a colonoscopy or sigmoidoscopy. In some cases, surgery may be needed to obtain a tissue sample^[10].

Laboratory specialists examine the tissue under a microscope to determine whether cancer cells are present. They also analyze how quickly the cells are growing and perform other tests to provide more detailed information about the cancer. This helps your healthcare team understand your specific situation and plan the most appropriate treatment^[10].

Blood Tests

While blood tests are not used to diagnose colorectal cancer on their own, they provide valuable information about your overall health. Blood tests can show how well your kidneys and liver are working, and they can detect low levels of red blood cells, which might indicate bleeding caused by colorectal cancer^[10].

Some colorectal cancers produce a protein called carcinoembryonic antigen, or CEA. Doctors can track CEA levels in your blood over time. If CEA was high at diagnosis and then decreases after treatment, this suggests the treatment is working. If CEA levels rise later, it may signal that the cancer has returned. However, if your CEA was normal at diagnosis, it usually isn’t helpful for detecting whether cancer has come back^[10].

Imaging Tests

Several imaging tests help doctors see inside your body without surgery. Computed tomography, or CT scan, uses X-rays and a computer to create detailed, three-dimensional images of your abdomen and other body parts. CT scans can show whether cancer has spread beyond the colon or rectum to other organs^[10].

Ultrasound uses sound waves to create pictures of organs and structures inside your body. A pelvic or abdominal ultrasound may be used to examine the area around the colon and rectum^[8].

Magnetic resonance imaging, or MRI, uses powerful magnets and radio waves to create detailed images of soft tissues in your body. MRI can be particularly useful for examining the rectum and surrounding structures in detail.

Imaging tests are typically done after cancer has been diagnosed to determine how far it has spread. This process is called staging. Staging helps doctors understand whether cancer is limited to the colon or rectum or has reached nearby lymph nodes or distant organs like the liver or lungs^[10].

Diagnostics for Clinical Trial Qualification

When patients are being considered for enrollment in clinical trials for colorectal cancer, specific diagnostic tests and criteria are typically required. Clinical trials are research studies that test new treatments or new ways of using existing treatments. They follow strict guidelines to ensure safety and to produce reliable results.

Most clinical trials require confirmation that a patient has colorectal cancer through a biopsy. The tissue sample must be examined by a pathologist who confirms the presence of cancer cells and provides details about the type and characteristics of the cancer. This is a standard requirement because trials need to be certain about what disease they are studying^[10].

Staging information is also typically required for clinical trial enrollment. Doctors use imaging tests such as CT scans, MRI, or other procedures to determine the stage of the cancer—meaning how large the tumor is, whether it has spread to lymph nodes, and whether it has reached distant organs. Different clinical trials may focus on specific stages of cancer, so accurate staging is essential for matching patients to the right studies^[10].

Blood tests are commonly part of the qualification process for clinical trials. These tests assess overall health, including how well your kidneys, liver, and bone marrow are functioning. Clinical trials often have specific requirements for blood counts and organ function to ensure that participants can safely tolerate the experimental treatment being tested^[10].

Some trials require testing for specific genetic changes or biomarkers in the cancer cells. For example, certain colorectal cancers have mutations in genes like BRAF or abnormalities in proteins that affect how the cancer grows. Knowing your biomarker status helps doctors understand which treatments might work best for you. About 5% of colorectal cancers are linked to inherited conditions like Lynch syndrome or familial adenomatous polyposis, and testing for these conditions may be part of trial eligibility^[7][8].

A physical exam and assessment of your overall health and daily functioning are also standard requirements. Clinical trials want to ensure that participants are healthy enough to participate and can comply with the study schedule, which may include frequent visits, additional tests, and careful monitoring.

Some trials focus on cancers that have specific characteristics, such as being resistant to standard chemotherapy or having returned after previous treatment. In these cases, doctors may review your treatment history and perform additional diagnostic tests to confirm that you meet the trial’s criteria.

CEA blood tests may also be part of the diagnostic workup for clinical trials. If your cancer produces this protein, tracking CEA levels can help researchers monitor how well the experimental treatment is working and whether the cancer is responding^[10].

It’s important to understand that each clinical trial has its own specific set of diagnostic requirements, and not every patient will qualify for every trial. Your healthcare team can help you understand which tests are needed and what the results mean for your eligibility. Participating in a clinical trial can provide access to new treatments that are not yet widely available, and your participation also helps advance medical knowledge that may benefit future patients.