Fecal Occult Blood Test for Colorectal Cancer Screening
For average-risk adults aged 45-75, perform annual high-sensitivity guaiac-based fecal occult blood testing (gFOBT) or fecal immunochemical testing (FIT), with all positive results requiring timely follow-up colonoscopy within 12 months.
Who Should Be Screened
Begin screening at age 45 for all average-risk adults, though the evidence is stronger for starting at age 50 1, 2. The American Cancer Society's 2018 recommendation to start at 45 is a "qualified" recommendation, while screening from age 50 onward is a "strong" recommendation 1. Average-risk means:
- No personal history of colorectal cancer or adenomatous polyps
- No family history of colorectal cancer
- No inflammatory bowel disease
- No hereditary cancer syndromes (Lynch syndrome, familial adenomatous polyposis)
Continue screening through age 75 for adults in good health with life expectancy >10 years 1, 3, 2.
Ages 76-85: Individualize decisions based on prior screening history, overall health status, and life expectancy. Those never previously screened are more likely to benefit 1, 2.
Age 86 and older: Do not screen—harms outweigh benefits 4.
How to Perform the Test
Test Selection
- High-sensitivity guaiac-based FOBT (gFOBT): Annually
- Fecal immunochemical test (FIT): Annually
FIT has superior sensitivity and specificity compared to unrehydrated gFOBT 5. FIT sensitivity for colorectal cancer is approximately 81% versus 13-25% for unrehydrated gFOBT 5. However, both have demonstrated mortality reduction in randomized trials for gFOBT 3.
Critical Performance Requirements
- Use voided stool samples only—never samples obtained by digital rectal examination 3
- Collect specimens from consecutive bowel movements (typically 2 samples per movement across 3 different days for gFOBT) 5
- Dietary restrictions for gFOBT: Eliminate citrus fruits, juices, and vitamin C for 3 days prior to and during collection 5
- No dietary restrictions needed for FIT
Common Pitfall
The American College of Physicians recommends testing every 2 years rather than annually 3, which differs from other major guidelines. However, the American Cancer Society and most other organizations recommend annual testing 1. Annual testing provides better sensitivity given the intermittent bleeding pattern of colorectal cancers and polyps.
How to Interpret Results
Positive Test
Any positive result mandates timely colonoscopy regardless of the quantitative value 1. This is non-negotiable.
- Timing: Perform colonoscopy within 12 months of positive result 6
- Higher quantitative FIT values correlate with higher cancer risk but do not eliminate the need for colonoscopy 6:
- FIT 20-39.9 μg/g: 0.9% positive predictive value for cancer
- FIT ≥200 μg/g: 6.7% positive predictive value for cancer
- Neoplastic findings occur across the entire spectrum of FIT values 6
Test Performance Characteristics
Understand the limitations 5:
- True positive rate: ~50% (meaning 50% of cancers are detected)
- False positive rate: 5-10%
- False negative rate: ~50% (half of cancers are missed)
- True negative rate: 90-95%
Interval Cancers
High interval cancer rates are a major limitation 5. In UK and Danish trials using unrehydrated gFOBT:
- 236 cancers detected by positive test, but 236 interval cancers developed after negative tests (UK trial)
- 120 cancers detected, but 146 interval cancers after negative tests (Danish trial)
This underscores why regular repeated testing is essential—a single negative test does not rule out cancer.
Expected Outcomes
Mortality Benefit
Biennial unrehydrated gFOBT screening reduces colorectal cancer mortality by:
No reduction in all-cause mortality has been demonstrated 3.
The average time to prevent 1 death from colorectal cancer is 10.3 years for 1000 patients screened 3, which explains why screening should stop when life expectancy falls below 10 years.
Detection Rates
For advanced lesions (cancer or advanced adenomas):
- Detection rate: 2.4-3.1% depending on cutoff used 7
- Number needed to scope: 2.0-2.3 colonoscopies to find one advanced lesion 7
Patient Counseling Points
Discuss the tradeoffs 3:
- Benefits: Small but real reduction in colorectal cancer death; non-invasive home-based test
- Harms:
- High false-negative rate (50% of cancers missed)
- False-positive results leading to unnecessary colonoscopy (5-10%)
- Colonoscopy carries 0.3% risk of serious complications (bleeding, perforation, cardiovascular events) 5
- Requires annual commitment to repeat testing
Patient preference matters in choosing between stool-based tests and structural examinations (colonoscopy, CT colonography, flexible sigmoidoscopy), but the choice must be based on high-quality accessible options 1.