What is the false negative rate of Fecal Occult Blood Test (FOBT)?

False Negative Rate of Fecal Occult Blood Test (FOBT)

The false negative rate of FOBT is approximately 50%, with studies showing that up to half of colorectal cancers may be missed by a single FOBT screening. 1, 2

Types of FOBT and Their Accuracy

Guaiac-based FOBT (gFOBT)

• Sensitivity for detecting colorectal cancer: 30-50% for a single test 3
• False negative rate: 5.5 per 1000 persons screened (CI, 2.8 to 8.2) 1
• Specificity: 87-96% 1
• High-sensitivity gFOBT (HSgFOBT) sensitivity: 62-79% 1

Fecal Immunochemical Test (FIT)

• Sensitivity for detecting colorectal cancer: 73-88% 1
• False negative rate: 0.69 per 1000 persons screened (CI, -0.02 to 1.4) 1
• Specificity: 91-96% 1
• Median sensitivity: 81.5% (range 53.3-100%) 1

Clinical Implications of False Negatives

False negative FOBT results can have serious consequences:

1. Increased mortality risk: Patients with false negative FOBTs who later develop colorectal cancer may have a threefold higher prevalence of metastatic disease (22.2% vs 7.5%) and significantly higher all-cause mortality (31.6% vs 8.8%) compared to those whose tests eventually convert to positive 4

2. Delayed diagnosis: In one study, 62% of patients with advanced colorectal cancer (Duke's C or D) had initially tested FOBT negative 5

3. False sense of security: A negative FOBT may provide false reassurance, as the 10-year cumulative risk for progression from polyps to clinically detectable colon cancer is about 10% 1

Factors Affecting FOBT Accuracy

• Test type: FIT has higher sensitivity and specificity than traditional gFOBT 1
• Sampling method: Single-sample FOBT collected during digital rectal examination has extremely poor sensitivity (only 4.9% for advanced neoplasia) 3
• Proper collection: Requires multiple samples (typically 2 samples from each of 3 consecutive bowel movements) 3
• Dietary factors: Affect guaiac-based tests but not immunochemical tests 3
• Cancer stage: Sensitivity increases with more advanced cancer stages:
  • T1 cancers: 38.5% detection
  • T2 cancers: 75.0% detection
  • T3 cancers: 78.9% detection
  • T4 cancers: 79.2% detection 6

Recommendations to Mitigate False Negatives

1. Use FIT over gFOBT when possible: FIT has consistently shown higher sensitivity for colorectal cancer detection 1, 3

2. Ensure proper testing protocol: Follow manufacturer's instructions for sample collection and avoid single-sample testing 3

3. Maintain regular screening intervals: Annual testing with high-sensitivity tests is recommended to compensate for the high false negative rate 1

4. Follow up positive results with colonoscopy: Any positive FOBT result should be followed by diagnostic colonoscopy, not repeat FOBT 1, 3

5. Consider the limitations: Be aware that FOBT is more effective at detecting cancer than adenomas, with adenoma detection rates being significantly lower 1, 6

Conclusion

The high false negative rate of FOBT (approximately 50%) represents a significant limitation of this screening method. While regular FOBT screening has been shown to reduce colorectal cancer mortality by 15-33% 3, the substantial false negative rate means that many cancers may be missed, particularly early-stage lesions. FIT offers improved sensitivity over traditional gFOBT but still misses a significant proportion of colorectal cancers.