What is the sensitivity and specificity of the occult blood stool test, specifically the immunochemical Fecal Occult Blood Test (FOBT) (iFOBT)?

Sensitivity and Specificity of Immunochemical Fecal Occult Blood Tests (iFOBT)

The immunochemical Fecal Occult Blood Test (iFOBT) has a sensitivity of approximately 79% (range 69-86%) for colorectal cancer detection and a specificity of approximately 94% (range 92-95%), making it superior to guaiac-based tests for cancer detection while maintaining excellent specificity. 1

Test Performance Characteristics

iFOBT Performance for Colorectal Cancer

• Sensitivity:

  • Meta-analysis of 19 studies shows pooled sensitivity of 79% (95% CI: 69-86%) 1
  • Range across studies: 50-100% depending on brand and cut-off values 1
  • Recent large US study using FDA-cleared OC FIT CHEK showed 74% sensitivity 1

• Specificity:

  • Pooled specificity of 94% (95% CI: 92-95%) 1
  • Range across studies: 85-97% 1
  • Generally higher than high-sensitivity guaiac tests 1

iFOBT Performance for Advanced Adenomas

• Sensitivity:

  • Significantly lower than for cancer detection
  • Range: 24-42% across studies 1
  • Pooled estimate approximately 28% 1

• Specificity:

  • Remains high at 87-97% 1

Comparison with Guaiac-Based Tests (gFOBT)

iFOBT demonstrates superior performance compared to traditional gFOBT:

Test Type	Sensitivity for Cancer	Specificity for Cancer
iFOBT	73-84%	89-95%
High-sensitivity gFOBT (Hemoccult SENSA)	64-79%	86-87%
Standard gFOBT (Hemoccult II)	13-37%	97-98%

Key advantages of iFOBT over gFOBT:

• Higher sensitivity for cancer detection 1
• No dietary restrictions required before testing 1
• Better patient compliance 1
• Quantitative results possible with some brands 1
• Automated processing available 2

Factors Affecting Test Performance

Several factors influence the sensitivity and specificity of iFOBT:

1. Cut-off threshold: Sensitivity can be adjusted by altering the hemoglobin threshold for a positive result, with lower thresholds increasing sensitivity but decreasing specificity 1

2. Number of samples:

   • Two-sample testing appears to provide optimal balance between sensitivity and cost-effectiveness 3
   • Single-sample testing may miss intermittent bleeding 1

3. Brand variations:

   • Different commercial iFOBT brands show varying performance
   • OC-Sensor and OC FIT CHEK demonstrate consistently good performance 1

4. Cancer stage:

   • Higher sensitivity for more advanced stages
   • Sensitivity by Dukes' stage: 71.4% (Stage A), 88.0% (Stage B), 96.7% (Stage C/D) 4

5. Cancer location:

   • Slightly better detection for distal colon/rectal cancers (92.6%) compared to proximal colon cancers (84.2%) 4

Clinical Implications and Limitations

Benefits

• Non-invasive screening option with good sensitivity and specificity
• Reduces colorectal cancer mortality when implemented properly 1
• Higher participation rates than endoscopic screening methods 1
• No dietary restrictions required 1

Limitations

• Less sensitive for early-stage cancers and advanced adenomas 1, 4
• Cannot detect non-bleeding lesions 1
• Requires repeat testing at regular intervals (annually) 1, 2
• All positive tests must be followed by diagnostic colonoscopy 1, 2

Common Pitfalls to Avoid

1. Inadequate follow-up: Nearly one-third of physicians incorrectly follow up a positive test with repeat stool testing rather than colonoscopy 2

2. Single in-office testing: Sensitivity is extremely low (4.9% for advanced neoplasia) and should never be used for screening 2

3. Delayed colonoscopy: Colonoscopy should be performed within 3-6 months after a positive test; delays beyond 6 months increase risk of advanced disease 2

4. Misinterpreting negative results: A negative test does not guarantee absence of colorectal cancer and requires annual repetition 2

Conclusion

The immunochemical Fecal Occult Blood Test (iFOBT) offers significant advantages over traditional guaiac-based tests for colorectal cancer screening, with higher sensitivity while maintaining excellent specificity. When properly implemented with appropriate follow-up of positive results, it can effectively contribute to reducing colorectal cancer mortality.