Colorectal Cancer Screening Test: Purpose, Test Details & Results

Colorectal cancer (CRC) is a leading cause of cancer-related deaths worldwide, but early detection through screening can make a significant difference in outcomes. Understanding the purpose, available screening tests, and what results mean is crucial for patients and healthcare providers alike. This article dives into the why, how, and what-next of colorectal cancer screening, empowering you to make informed decisions and understand the journey from screening to follow-up.

Purpose of Colorectal Cancer Screening Test

Screening for colorectal cancer is not just about finding cancer; it's about preventing it and catching it early when treatment is most effective. Many people feel anxious about screening, but understanding its purpose can help make the process more approachable and meaningful.

Table 1: Core Purposes of Colorectal Cancer Screening

Why Screen for Colorectal Cancer?

Colorectal cancer typically develops slowly from benign polyps in the colon or rectum. By detecting these polyps or early-stage cancers before symptoms arise, screening can prevent cancer entirely or catch it when it's most treatable 1 2 5 9. Most screening programs target adults starting at age 45–50, as risk increases with age 7 9.

How Screening Saves Lives

Screening reduces both the incidence and mortality of CRC:

• Incidence reduction: By finding and removing polyps before they turn cancerous 1 5.
• Mortality reduction: By detecting cancers earlier, when survival rates are much higher 1 2 5 9.
• Population impact: Organized screening programs have led to significant declines in CRC deaths in countries with high participation 3 4 5.

Individualized and Population Health Approaches

Screening recommendations can be tailored:

• Average-risk adults: Most guidelines recommend starting regular screening at age 45 or 50 7 9.
• High-risk individuals: Those with family history or certain genetic conditions may begin screening earlier or use more sensitive methods 7 9.
• Life expectancy consideration: Screening usually continues up to age 75, with individualized decisions for those aged 76–85 9.

Colorectal Cancer Screening Test Details

Today, several screening tests offer different balances of sensitivity, specificity, convenience, and risk. Understanding the details helps you and your healthcare provider choose the best test for you.

Table 2: Overview of Colorectal Cancer Screening Tests

Stool-Based Tests

Fecal Immunochemical Test (FIT):

• Detects hidden blood in the stool using antibodies specific for human hemoglobin.
• High specificity (94%), good sensitivity (74%) for cancer detection 2 4 7 8 9 10.
• Simple, non-invasive, and preferred for mailed outreach programs 8.
• Needs annual repetition.

Guaiac-based Fecal Occult Blood Test (gFOBT):

• Detects blood via a chemical reaction on a paper card.
• Lower sensitivity (50–75%) than FIT, but highly specific 1 4 5 9 10 12.
• Requires dietary restrictions to avoid false positives.
• Needs annual testing.

Multitarget Stool DNA (mt-sDNA):

• Combines DNA markers and hemoglobin detection.
• Higher sensitivity (92% for cancer) but more false positives (specificity 86.6%) 6 9.
• Performed every 3 years.

Visual/Structural Tests

Colonoscopy:

• Visualizes, biopsies, and removes polyps throughout the colon.
• Considered the gold standard for detection and prevention 5 7 9.
• Performed every 10 years if results are normal.
• Requires bowel preparation and sedation.
• Risks include bleeding, perforation (rare) 2.

Flexible Sigmoidoscopy:

• Examines the lower colon (sigmoid and rectum).
• Less invasive, no sedation required.
• Reduces CRC incidence by 18% and mortality by 28% 2 5.
• Every 5–10 years.

CT Colonography:

• Uses CT imaging to visualize the colon.
• Similar sensitivity to colonoscopy for larger polyps (≥6mm) 2 7 9.
• Every 5 years.
• Exposes patient to low-dose radiation.

Emerging and Alternative Tests

Blood-based Tests:

• Currently under investigation; one FDA-approved (Epi procolon®) in the US 3 8.
• Potentially increases participation but lower sensitivity compared to stool-based tests.

Other Methods:

• Colon capsule endoscopy and MR colonography are being researched 8.

Choosing the Right Test

• Tier 1 (most recommended): Colonoscopy and annual FIT 7.
• Tier 2: CT colonography (every 5 years), multitarget stool DNA (every 3 years), flexible sigmoidoscopy 7 9.
• Patient preference, test availability, and risk factors drive the choice.
• Positive results from non-colonoscopy tests should always be followed up with colonoscopy 7 9.

Colorectal Cancer Screening Test Results & Follow-Up

Understanding your test results, what they mean, and what comes next is the final—and often most anxiety-provoking—step in the screening process. Prompt follow-up is essential to ensure the benefits of screening are fully realized.

Table 3: Interpreting Colorectal Cancer Screening Results

What Do Screening Results Mean?

• Negative result: Most people will have a negative result, meaning no hidden blood or abnormal DNA was found. This does not guarantee absence of cancer but means risk is very low; continue screening at recommended intervals 12 9.
• Positive result (stool-based or blood test): Indicates blood or abnormal DNA in the stool, which could be from cancer, polyps, or non-cancer causes (like hemorrhoids). A positive result requires diagnostic colonoscopy 6 7 9.

Importance of Follow-Up After Positive Tests

• Colonoscopy is essential after any positive non-invasive screening test to confirm the presence of cancer or polyps 7 9 13.
• Timely follow-up ensures that cancers or precancerous lesions are not missed.
• Unfortunately, not all patients receive recommended follow-up; for example, only about 59% of patients with positive stool tests had a follow-up colonoscopy in one study 13.

Colonoscopy Results and Next Steps

• If polyps are found: They can often be removed during the procedure, preventing cancer development 2 5 7 9.
• If cancer is found: Biopsy and staging are performed, and treatment options are discussed with the patient.
• If normal: Return to routine screening at the recommended interval for your risk group 7 9 12.

Understanding Test Sensitivity and Specificity

• Sensitivity is the ability to detect disease when present. FIT and stool DNA tests have higher sensitivity than older gFOBT 2 6 10.
• Specificity is the ability to avoid false positives. FIT and gFOBT are highly specific, but multitarget stool DNA has slightly lower specificity, leading to more false positives 6 10 12.
• No test is perfect—some polyps or cancers may be missed, especially with single-round testing, which is why adherence to repeat screening is essential 12.

Psychological and Practical Considerations

• False positives can cause anxiety and lead to unnecessary procedures, but they are balanced against the benefit of early detection 1 6.
• False negatives are possible, so regular screening is important even after a negative result 12.
• Harms of screening include rare risks like bleeding or perforation from colonoscopy, and the psychosocial impact of waiting for results or undergoing procedures 2 5.

Conclusion

Colorectal cancer screening is a critical tool that saves lives by preventing cancer or catching it early. Understanding the purpose, available tests, and what results mean can help you make informed decisions about your health.

Key Points:

• Screening prevents and detects colorectal cancer early, reducing deaths and incidence 1 2 5 9.
• Multiple test options exist (FIT, gFOBT, stool DNA, colonoscopy, sigmoidoscopy, CT colonography), each with advantages and limitations 4 5 7 8 9.
• Choice of test depends on individual risk, preferences, and resource availability 7 9.
• All positive non-colonoscopy results require follow-up colonoscopy to confirm diagnosis and enable treatment 6 7 9 13.
• Regular screening and timely follow-up are essential for best results; adherence makes a difference 12 13.
• Emerging technologies and blood-based tests may offer new screening options in the future, potentially increasing participation 3 8.

Discuss with your healthcare provider which colorectal cancer screening test is right for you, and remember: regular screening could save your life.