What is the FIT Test for Colon Cancer?
The fecal immunochemical test (FIT) is a non-invasive stool-based screening test that detects hidden blood in stool by using antibodies specific to human hemoglobin, making it highly effective for colorectal cancer screening with superior performance compared to older guaiac-based tests. 1
How FIT Works
FIT uses immunochemical technology (latex immunoturbidimetry) to detect human hemoglobin in stool samples. Unlike older guaiac fecal occult blood tests (gFOBT), FIT specifically targets human hemoglobin, which provides several key advantages:
- No dietary restrictions needed - patients don't need to avoid red meat, vitamin C, or other foods 1
- No medication adjustments required - can be performed while taking aspirin or other medications 1
- Single sample collection - only one stool sample is needed per testing cycle, compared to three samples for older tests 2
- Higher sensitivity - FIT detects 75-100% of colorectal cancers compared to 37.5-61.5% for gFOBT 1
Clinical Performance
Cancer Detection
FIT demonstrates strong performance for colorectal cancer detection:
- Overall sensitivity for CRC: 68-100% depending on cutoff level 1, 3
- Stage-specific sensitivity: 68% for stage I, 92% for stage II, 82% for stage III, and 89% for stage IV cancers 3
- Location matters: Better detection of left-sided and rectal cancers (58% mortality reduction) compared to right-sided cancers 4
Real-World Impact
A 2024 nested case-control study of over 2 million people demonstrated that completing one or more FIT screenings was associated with a 33% lower risk of death from colorectal cancer 4. The protective effect was particularly strong for left colon and rectal cancers (42% mortality reduction) and was consistent across racial and ethnic groups 4.
Recommended Testing Strategy
Based on the 2017 US Multi-Society Task Force guidelines 1:
- Frequency: Annual testing (one sample per year)
- Cutoff threshold: 20 μg Hb/g or lower for defining positive results
- Test type: Quantitative FIT preferred over qualitative
- Sample collection: Use spontaneously passed stool, NOT digital rectal exam samples
- Positive result follow-up: Colonoscopy is mandatory for any positive FIT
Important Caveats
FIT has limitations you must understand:
- Misses approximately one-third of stage I cancers 3
- Less sensitive for advanced adenomas (23-33%) compared to cancers 1
- Requires annual repetition - this is not a one-time test
- Negative predictive value is excellent (99.95%), but sensitivity for early cancers is imperfect 5
Comparison to Other Screening Methods
FIT vs Colonoscopy
- Participation: FIT achieves 34-62% participation vs 25-27% for colonoscopy 1
- Detection: Colonoscopy detects more advanced adenomas (6.3% vs 1.1%), but similar cancer rates 1
- Trade-off: To detect one advanced neoplasm, you need to invite 264 people with FIT vs 53 with colonoscopy 1
FIT vs Older Stool Tests
FIT is definitively superior to gFOBT:
- 2.3 times better at detecting advanced neoplasia 1
- 16% higher adherence rates 1
- No dietary restrictions make it more practical 1
Program Implementation
For screening programs to succeed, the 2017 guidelines recommend these quality targets 1:
- 60% completion rate among those offered testing
- <5% of returned FITs cannot be processed by lab
- Colonoscopy completion for positive results should be tracked
Sequential Testing Approach
In US practice, offering colonoscopy first, then FIT to those who decline, improves overall screening participation by 19-25% compared to offering only one option 1.
Double-FIT Strategy
Recent evidence suggests that repeating FIT when the first test is negative can reduce missed cancers by 50-75% 6. At a 10 μg Hb/g threshold, double-FIT achieves 94-98% sensitivity for colorectal cancer in mixed-risk and symptomatic populations 6. This strategy increases test positivity by 3-9 percentage points but substantially reduces missed cancers 6.
Bottom Line for Clinical Practice
FIT is a highly effective, patient-friendly screening tool that reduces colorectal cancer mortality when used annually. While colonoscopy detects more adenomas per person screened, FIT's superior participation rates mean more cancers are prevented at the population level 1, 7. The test requires no preparation, works with a single stool sample, and when positive, mandates colonoscopy follow-up. Its main weakness is missing early-stage cancers, which is why annual repetition is essential 3.