This study is designed to evaluate the utility of Narrow Band Imaging (NBI) compared with High Definition White Light colonoscopy (WLE) in subjects with serrated lesions who do not fulfill the diagnostic criteria of Serrated Polyposis Syndrome (SPS).

Colorectal cancer (CRC) is the second leading cause of cancer death in western countries. Conventional polyps were considered the precursor lesions of all cases of sporadic colon cancer. Recently, serrated lesions and especially the Sessile Serrated Adenoma (SSA), are responsible of interval CRC between 20% to 35% of all CRC cases. These polyps are difficult to identify at endoscopy because they are located in the right colon, they are sessile or flat morphology and are pale color with mucus capping.

According to the WHO, SPS is defined with one of the following criteria: (1) at least 5 serrated polyps proximal to the sigmoid colon, 2 of which are greater than 10 mm in diameter; (2) any number of serrated polyps occurring proximal to the sigmoid colon in an individual who has a first-degree relative with serrated polyposis; or (3) more than 20 serrated polyps of any size distributed throughout the colon. Therefore, patients with SPS are considered to be at increased risk of CRC. Considering the substantial risk of polyp recurrence, it is mandatory to follow up an annual surveillance.

Narrow-Band Imaging (NBI, Olympus) selectively uses certain wavelengths of the visible light leading to a shift in the excitation spectrum towards blue light. Blood vessels will appear dark, allowing an improved visibility and identification of the surface and vascular structures. In contrast to conventional chromoendoscopy, it is easily activated by pressing a button on the endoscope. A pilot study in patients with SPS showed significantly lower polyp miss rate with NBI compared with WLE. Furthermore, the European Society of Gastrointestinal Endoscopy (ESGE) has recently published the first Guideline of Advanced Endoscopic Imaging for the detection and differentiation of colorectal neoplasia and recommends conventional chromoendoscopy or NBI in patients with SPS (strong recommendation, low quality evidence).

The hypothesis is that NBI could improve the detection rate of serrated polyps compared with WLE in patients who do not accomplish the SPS criteria.

The investigators will perform a randomised, cross-over trial of tandem colonoscopy using NBI and WLE. The main goal is to compare the rate of detected polyps between both techniques and, if it is necessary, reassessing the diagnosis for an appropriate surveillance interval.

Inclusion Criteria:

• Patients over 50 years old who accept CRC screening colonoscopy
• Patients with a basal colonoscopy findings: ≥1 serrated polyps proximal to the sigmoid colon, which are greater than ≥10mm in diameter; or ≥3 serrated polyps proximal to the sigmoid colon

Exclusion Criteria:

• Diagnosis of a CRC in the basal colonoscopy
• Subjects with other types of histology polyps
• Subjects who neglect to follow-up
• Subjects who do not accept informed consent
• Subjects with high risk of perforation or complications due to sedation, including patients with comorbidities (ASA IV-V)
• Inadequate bowel preparation for colonoscopy (defined by Boston Bowel Preparation Score (BBPS): ≤ 5 total points; or 0-1 points in any of the 3 segments of the colon)