From the Guidelines
A positive Fecal Occult Blood Test (FOBT) result in colorectal cancer screening is indicated by the presence of blood in the stool, which may be caused by colorectal cancer, polyps, or other conditions, and requires a diagnostic workup with colonoscopy to examine the entire colon and rule out the presence of cancer or advanced neoplasia. The indicators for a positive FOBT result include the detection of occult blood in stool, which can be identified through guaiac-based fecal occult blood tests (gFOBT) or fecal immunochemical tests (FIT) 1. According to the American College of Physicians, suggested screening tests and intervals are fecal immunochemical testing or high-sensitivity guaiac-based fecal occult blood testing every 2 years, colonoscopy every 10 years, or flexible sigmoidoscopy every 10 years plus fecal immunochemical testing every 2 years 1.
Key Points to Consider
- A positive FOBT result suggests that blood is present in the stool, which may be caused by colorectal cancer, polyps, or other conditions such as hemorrhoids, inflammatory bowel disease, or anal fissures.
- The sensitivity of FOBT for detecting colorectal cancer varies, but fecal immunochemical testing is associated with better sensitivity and specificity than gFOBT 1.
- When a FOBT test returns positive, follow-up with a diagnostic colonoscopy is essential and should be scheduled promptly to determine the source of bleeding.
- It's essential to understand that a positive FOBT is not a definitive diagnosis of cancer but rather an indication that further investigation is needed.
Recommendations for Practice
- Clinicians should select the colorectal cancer screening test with the patient on the basis of a discussion of benefits, harms, costs, availability, frequency, and patient preferences 1.
- Positive findings on stool-based tests should be confirmed with direct visualization tests, such as colonoscopy, to rule out the presence of cancer or advanced neoplasia 1.
From the Research
Indicators for a Positive Fecal Occult Blood Test (FOBt) Result
The indicators for a positive Fecal Occult Blood Test (FOBt) result in colorectal cancer screening include:
- Fecal hemoglobin concentration: Studies have shown that the risk of advanced colorectal neoplasia (ACRN) and colorectal cancer (CRC) increases with increasing fecal hemoglobin concentration 2, 3, 4
- Time to colonoscopy after a positive FIT result: Delaying colonoscopy after a positive FIT result may increase the risk of ACRN and CRC 2
- FIT cutoff value: The optimal FIT cutoff value for detecting advanced adenomas and CRC is still a topic of debate, but studies suggest that a cutoff value of 400 ng Hb/mL may be effective 3
- Age: The positive predictive value (PPV) of FIT for advanced adenomas and CRC may vary by age, with higher PPV values observed in older age groups 5
Fecal Hemoglobin Concentration and Colorectal Cancer Risk
The relationship between fecal hemoglobin concentration and colorectal cancer risk is supported by several studies:
- A study published in 2019 found that ACRN risk increased with increasing fecal hemoglobin concentration, with a significant increase in risk observed at concentrations ≥1000 ng Hb/mL 2
- A study published in 2024 found that the size of colorectal lesions was positively correlated with FIT values, with higher FIT values observed in patients with larger lesions 3
- A study published in 2020 found that reducing the abnormal FIT result cutoff value to ≥10 μg hemoglobin/g feces may increase detection of advanced neoplasia, but also increases the proportion of patients requiring a diagnostic colonoscopy 4
Clinical Implications
The indicators for a positive FOBt result have important clinical implications:
- Patients with positive FIT results, especially those with higher fecal hemoglobin levels, should undergo timely follow-up colonoscopy to reduce the risk of ACRN and CRC 2
- The optimal FIT cutoff value and screening strategy may vary depending on the population being screened and the available resources 3, 4
- Healthcare providers should consider the age and fecal hemoglobin concentration of patients when interpreting FIT results and making decisions about further testing and treatment 5