The goal of this clinical trial is to evaluate the diagnostic value of fecal immunochemical tests (FIT) in identifying colorectal cancer (CRC) among Vietnamese individuals presenting with lower gastrointestinal symptoms.

The main questions it aims to answer are:

1. Does a quantitative FIT with a positive threshold of 10 µg/g effectively diagnose CRC in symptomatic Vietnamese individuals using one and two stool samples?
2. Does combining FIT results with the Asia-Pacific Colorectal Screening (APCS) score improve the diagnostic accuracy for CRC in symptomatic Vietnamese individuals?
3. What is the optimal quantitative FIT threshold for diagnosing CRC and excluding organic colorectal diseases (advanced colorectal polyps, inflammatory bowel disease, ischemic colitis, ...)?
4. How can a standardized protocol for FIT implementation in Vietnamese symptomatic individuals be developed?
5. What are the knowledge and attitudes related to CRC screening among Vietnamese patients with lower gastrointestinal symptoms?
6. What is the acceptability of FIT as a diagnostic tool among Vietnamese patients with lower gastrointestinal symptoms?

Researchers will evaluate the diagnostic performance of FIT with and without the APCS score to determine the optimal approach for identifying patients requiring colonoscopy.

Participants will:

• Provide two stool samples for quantitative FIT.
• Complete a questionnaire assessing clinical and demographic factors, as well as their knowledge, attitudes toward CRC screening, and acceptability of FIT.
• Undergo diagnostic colonoscopy to confirm the presence or absence of colorectal diseases.

This study aims to enhance CRC diagnostic strategies and establish FIT as a widely acceptable and non-invasive diagnostic tool in resource-limited settings.

Inclusion Criteria: In order to be eligible to participate in this study, an individual must meet all of the following criteria:

• Aged 40 - 75 years.
• Informed consent available.
• Patients with lower GI symptoms (patients who experience at least one month of any lower GI symptoms: lower abdominal pain, constipation, diarrhea, changes in stool caliber, and abdominal bloating) who are scheduled for a first-time screening colonoscopy.

Exclusion Criteria: Individuals meeting any of the following exclusion criteria will be excluded from the study:

1. History of chronic inflammatory bowel disease, diverticular disease of the colon, colorectal surgery, polypectomy, colorectal cancer, artificial heart valves, or vascular graft surgery.
2. Previous colorectal diagnostic imaging, including colonoscopy, sigmoidoscopy, computed tomography (CT) colonography, or barium enema.
3. Presence of alarm symptoms, including lower gastrointestinal bleeding (hematochezia or melena), an abdominal mass, or unexplained clinically significant weight loss (loss of more than 5 kg or 5-10% of body weight over the past 6 months).
4. Symptoms or signs of anorectal tumors or anal ulcers.
5. Acute diarrhea lasting ≤14 days.
6. Current use of aspirin, antiplatelet agents, anticoagulants, or nonsteroidal anti-inflammatory drugs (NSAIDs), as these may cause false-positive FIT results.
7. Diagnosis of hereditary colorectal cancer syndromes (e.g., Lynch syndrome or familial adenomatous polyposis).
8. Pregnancy, breastfeeding, or menstruation at the time of the study.
9. Contraindications to colonoscopy.
10. Incomplete colonoscopy due to failure to advance the scope to the cecum or inadequate bowel preparation (Boston Bowel Preparation Scale [BBPS] score <6 or a segment score <2).