FIT (Fecal Immunochemical Test) — Colorectal Cancer
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 21% (sensitivity 79%) |
| False-positive rate | 6% (specificity 94%) |
| Bayes factor (positive test) | 10× |
| Bayes factor (negative test) | 1/5× |
| Base rate | 0.5% for CRC in average-risk adults |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| Average-risk adult, age 45–75 | 0.5% | 10 × 0.5% = 5%0.5%× 105% | 1/5 × 0.5% = 0.1%0.5%÷ 50.1% |
| First-degree relative with CRC | 2% | 10 × 2% = 20%2%× 1020% | 1/5 × 2% = 0.4%2%÷ 50.4% |
Positive FIT is a strong signal, but the 21% miss rate limits rule-out power. Positive result warrants follow-up colonoscopy.
- + result: risk goes from identical twin (0.5%) to vegetarian (5%)
- − result: risk drops to appendicitis this year (0.1%)
Sources:
- Systematic review and meta-analysis, PMC, 2014. One-sample FIT in average-risk asymptomatic adults.