Colorectal Cancer (CRC) screening is a preventative health measure designed to detect abnormal growths or cancer in the colon and rectum before symptoms develop. The primary goal of a CRC test is to find precancerous polyps, which are small growths on the lining of the colon that can take years to become malignant. Identifying and removing these polyps during screening prevents cancer from forming. This process significantly reduces the risk of developing colorectal cancer and increases the chance of successful treatment if cancer is found early.
Why and When Colorectal Cancer Screening is Necessary
Screening is recommended because colorectal cancer often progresses silently, with early-stage disease causing no noticeable symptoms. By the time symptoms such as unexplained weight loss, persistent changes in bowel habits, or rectal bleeding appear, the cancer may be more advanced and complex to treat. Routine testing allows for intervention at a time when the disease is most vulnerable.
For individuals at average risk, the standard recommendation from medical task forces is to initiate screening at age 45. This guideline was recently lowered due to a rise in colorectal cancer rates among younger adults. Screening continues regularly until age 75, with decisions for those between 76 and 85 being individualized based on overall health and prior screening history.
Certain factors may necessitate starting the screening process earlier or undergoing more frequent testing. Individuals with a personal or strong family history of colorectal cancer or precancerous polyps are considered to be at higher risk. The presence of specific conditions, such as inflammatory bowel diseases or inherited genetic syndromes, also places a person in a high-risk category.
Categorizing the Different Screening Tests
Colorectal cancer screening tests fall into two main categories: non-invasive, stool-based tests and visual, structural examinations. The choice of test often depends on individual preference, risk level, and the recommended screening interval.
Stool-Based Tests (Non-Invasive)
Stool-based tests are completed at home and require minimal to no preparation, though they are primarily designed to detect signs of cancer rather than precancerous polyps. The Fecal Immunochemical Test (FIT) looks specifically for hidden human hemoglobin, which is a protein component of blood that may be shed by polyps or tumors. This test is typically performed annually for average-risk individuals.
The Guaiac-based Fecal Occult Blood Test (gFOBT) detects blood using a chemical reaction, requiring dietary restrictions before the test to avoid false-positive results from certain foods. The Multi-target Stool DNA test is a more advanced option that analyzes the stool sample for both altered DNA markers associated with cancer and for human hemoglobin. This test is recommended less frequently, usually every one to three years.
A positive result from any stool-based test indicates that follow-up is necessary. These screening methods cannot determine the source of the blood or altered DNA, so a diagnostic procedure is required.
Visual/Structural Exams (Invasive)
Visual exams allow a healthcare provider to physically examine the colon or rectum for polyps and other abnormalities. Colonoscopy is considered the most comprehensive option, as a flexible, lighted tube is used to view the entire length of the colon. If the results are clear, this examination is recommended once every 10 years.
Flexible Sigmoidoscopy is a similar procedure that only examines the lower third of the large intestine. Because it covers less area, it is generally recommended more frequently, every five years. CT Colonography, sometimes called a virtual colonoscopy, uses a computed tomography scan to create two- and three-dimensional images of the colon’s interior. This exam requires the same intensive bowel preparation as a colonoscopy and is typically repeated every five years.
Patient Preparation and Procedure for Diagnostic Testing
The most common diagnostic test following an abnormal screening result is a colonoscopy, which requires preparation. Preparation begins days before the appointment, often with a low-fiber diet to reduce the amount of residue in the digestive tract.
The day before the procedure, the patient must switch to a strict clear liquid diet, consuming only clear broths, water, and clear juices. This step is followed by the bowel preparation, a powerful liquid laxative solution that is ingested in a split-dose regimen. The goal of this preparation is to completely empty the colon, as any remaining stool can obscure the physician’s view and potentially lead to a missed finding.
On the day of the procedure, a patient is typically given deep sedation, ensuring they are comfortable and have no memory of the examination. The procedure usually takes less than an hour, depending on whether polyps need to be removed. In the recovery area, the sedation effects wear off quickly, though patients may feel groggy or experience mild bloating from the air used to inflate the colon during the exam. Due to the lingering effects of the sedative, a responsible adult must be present to drive the patient home, and the patient must avoid driving or making important decisions.
Understanding Test Results and Next Steps
A negative result from a screening test means no signs of disease were found, and the patient can return to a standard screening interval. This interval is typically annual for a FIT test, every one to three years for a Multi-target Stool DNA test, and once every 10 years after a clear colonoscopy.
If a non-invasive test like FIT or Stool DNA returns a positive result, the next required step is a diagnostic colonoscopy, as the stool test is a screen and not a final diagnosis. A positive result does not automatically mean cancer; the abnormality may be caused by benign conditions or polyps.
During a colonoscopy, if polyps are found, they are typically removed immediately in a procedure called a polypectomy. The follow-up surveillance schedule is determined by the size, number, and type of polyps removed. Low-risk findings may lead to a repeat colonoscopy in seven to ten years, while higher-risk polyps necessitate surveillance in three to five years. If a suspicious mass is found, a biopsy is taken. If cancer is confirmed, the patient is referred to a multidisciplinary team, including oncologists and surgeons, who use imaging scans to determine the stage and formulate a treatment plan.