Fecal occult blood testing: clinical value and limitations

Area of Science:

• Gastroenterology
• Oncology
• Diagnostic Medicine

Background:

• Occult blood in stool is a key indicator for colorectal cancer screening.
• Guaiac slide tests (e.g., Hemoccult II) are widely used but have limitations.
• Technical factors like slide rehydration significantly impact test performance.

Purpose of the Study:

• To evaluate the clinical performance and effectiveness of different stool occult blood tests for colorectal cancer screening.
• To analyze the impact of technical factors, such as slide hydration, on test sensitivity and specificity.
• To assess the mortality benefit and cost-effectiveness of occult blood screening programs.

Main Methods:

• Review of chemical (guaiac), heme-porphyrin, and immunological methods for occult blood detection.
• Analysis of clinical trial data on the sensitivity, specificity, and predictive value of hydrated and unhydrated Hemoccult tests.
• Evaluation of mortality reduction data associated with periodic occult blood screening.

Main Results:

• Unhydrated Hemoccult shows ~50% sensitivity and ~98% specificity for colorectal cancer; only 5-10% of positive tests are cancer-related.
• Hydrated Hemoccult increases sensitivity but drastically reduces specificity, leading to a ~2% positive predictive value for cancer.
• Periodic occult blood screening (biennial, unhydrated) yields a 15-18% colorectal cancer mortality benefit.

Conclusions:

• Stool occult blood screening provides a significant, albeit modest, mortality benefit for colorectal cancer.
• Poor specificity and low positive predictive values of current tests necessitate careful consideration of screening protocols and costs.
• Despite documented benefits, widespread implementation faces challenges due to compliance, cost, and ethical considerations.