What are the sensitivity and specificity of occult blood tests (FOBT - Fecal Occult Blood Test) in stools?

Sensitivity and Specificity of Fecal Occult Blood Tests (FOBTs)

Immunochemical fecal occult blood tests (iFOBTs) have superior sensitivity and specificity compared to guaiac-based tests (gFOBTs), with iFOBTs demonstrating approximately 79% sensitivity (range 69-86%) and 94% specificity (range 92-95%) for colorectal cancer detection. 1

Types of Fecal Occult Blood Tests and Their Performance

Guaiac-based FOBTs (gFOBTs)

• Sensitivity for colorectal cancer: 50-75% (95% CI: 0.09-1.0) 2
• Specificity for colorectal cancer: 96-98% (95% CI: 0.95-0.99) 2
• Sensitivity for advanced adenomas: Much lower at 6-17% (95% CI: 0.02-0.23) 2
• High-sensitivity gFOBTs perform better than standard gFOBTs 2

Immunochemical FOBTs (iFOBTs/FITs)

• Sensitivity for colorectal cancer: 74% (95% CI: 0.64-0.83) for OC-Sensor family 2
• Specificity for colorectal cancer: 94% (95% CI: 0.93-0.96) 2
• Sensitivity for advanced adenomas: 23% (95% CI: 0.20-0.25) 2
• No dietary restrictions required, unlike gFOBTs 1

Stool DNA-FIT (Combined Test)

• Highest sensitivity for colorectal cancer: 93% (95% CI: 0.87-1.0) 2
• Lower specificity than FIT alone: 84% (95% CI: 0.84-0.86) 2
• Better sensitivity for advanced adenomas: 43% (95% CI: 0.40-0.46) 2

Factors Affecting Test Performance

Anatomical Location

• iFOBTs show significantly lower sensitivity for proximal (right-sided) lesions compared to distal (left-sided) lesions
• Sensitivity for left-sided advanced neoplasia: 33% (95% CI: 26-41%) 3
• Sensitivity for right-sided advanced neoplasia: 20% (95% CI: 11-31%) 3
• This difference may be partly explained by adenoma morphology, with pedunculated lesions (more common on the left side) being more likely to bleed 3

Cancer Stage

• Sensitivity increases with advancing cancer stage
• For iFOBT: 50% for Dukes' stage A, 70% for Dukes' stage B, and 78.3% for Dukes' stages C or D 4
• Significantly higher sensitivity for stages III and IV compared to stages I and II 5

Test Handling and Storage

• iFOBT samples are stable for 21 days when refrigerated at 4°C 6
• Significant degradation occurs at higher temperatures (3.7% decay/day at 28°C) 6
• Proper sample collection and handling are critical for accurate results

Clinical Implications

1. Test Selection: iFOBTs generally offer better performance than gFOBTs, with higher sensitivity for cancer detection while maintaining excellent specificity 1

2. Testing Frequency: Annual testing is recommended due to the limited sensitivity of a single test, especially for early-stage cancers and advanced adenomas 2, 1

3. Follow-up Protocol: Any positive FOBT result must be followed by diagnostic colonoscopy 2, 1

4. Common Pitfalls:

   • Inadequate follow-up of positive results
   • Single in-office testing (instead of multiple samples)
   • Delayed colonoscopy after positive results
   • Misinterpreting negative results as definitive (false reassurance) 1

5. Test Threshold: For iFOBTs, sensitivity can be adjusted by altering the hemoglobin threshold, with lower thresholds increasing sensitivity but decreasing specificity 1

FOBTs are valuable screening tools for colorectal cancer, but their limitations must be understood, particularly their lower sensitivity for early-stage cancers and advanced adenomas, which necessitates regular repeated testing in screening programs.