No, FIT is NOT a Blood Test
FIT (fecal immunochemical test) is a stool-based test that detects human hemoglobin in feces, not blood. This is a critical distinction that affects how the test is ordered, collected, and interpreted.
What FIT Actually Tests
FIT uses antibodies to detect human globin (a protein component of hemoglobin) in stool samples 1, 2. The test specifically identifies blood that has passed through the gastrointestinal tract and appears in feces, making it useful for colorectal cancer screening.
Key Technical Characteristics:
- Sample type: Stool/feces collected at home by the patient
- Detection method: Immunochemical antibodies that react to human globin
- Specificity: Detects only human blood (unlike older guaiac tests that react to dietary sources) 1
- Target location: Lower gastrointestinal bleeding, as globin degrades in the upper GI tract 1, 3
How FIT Differs from Blood Tests
The confusion may arise because FIT detects blood (hemoglobin), but it does so by analyzing stool samples, not by drawing blood from a vein. Recent developments in colorectal cancer screening have introduced actual blood-based tests (like Guardant Shield), which are serum tests that detect circulating tumor DNA 4. These are fundamentally different from FIT.
Critical Distinctions:
- FIT: Patient collects stool at home, no venipuncture required
- Blood-based CRC screening tests: Require venipuncture, detect circulating biomarkers
- Collection burden: FIT requires less direct stool handling than older guaiac tests 1
Clinical Implications
Understanding that FIT is a stool test matters for:
- Patient counseling: Patients need clear instructions for home stool collection 3
- Screening frequency: Annual testing is recommended for FIT 2, 5
- Follow-up: Any positive FIT requires colonoscopy, not repeat stool testing 1, 3
- Dietary restrictions: Unlike guaiac tests, FIT does not require dietary modifications (no need to avoid red meat, vitamin C, etc.) 1, 2
Common Pitfall
Do not confuse FIT with emerging blood-based colorectal cancer screening tests. While both screen for colorectal cancer, FIT analyzes stool for occult blood, whereas newer serum tests (not yet widely recommended in guidelines) detect circulating tumor markers 4. The USPSTF currently does not include serum tests in their colorectal cancer screening recommendations due to limited evidence 2.
FIT remains a stool-based, non-invasive screening option that detects blood in feces through immunochemical methods, requiring annual testing and colonoscopy follow-up for positive results.