The investigators aimed to compare the efficacy for bowel cleansing of a split-dose regimen with a low morning dose of PEG solution (Asymmetric; 25% of the dose is given on the day of the procedure and 75% of the dose is given on the day before) with the standard split-dose regimen in patients undergoing screening and surveillance colonoscopy using a very low volume bowel preparation (1L PEG+Ascorbate). Consecutive outpatients undergoing screening and surveillance colonoscopy were enrolled in a randomized, single-blind, non-inferiority clinical trial. Patients were randomly assigned to: group A, asymmetric split dose regimen (0,75 L of PEG + ASC the day before and 0,25 L 2:30 hours before colonoscopy); group B, symmetric split dose regimen (0,5 L of PEG + ASC the day before and 0,5 L 3 hours before colonoscopy). Primary endpoint was the proportion of adequate bowel cleansing. Moreover, all patients filled in a validated, nurse-administered questionnaire assessing compliance, tolerability and safety of bowel preparation.

Prospective observational, single-blind study conducted in two endoscopy centers belonging to the same Local Health Authority. The two bowel preparation regimens under study (symmetric split and asymmetric split) will be compared in a population of patients undergoing screening for colorectal cancer.

Patient enrollment, product prescription, and the subsequent pre-endoscopic evaluation of safety, tolerability, acceptability, and compliance will be carried out by an assistant who will not be blinded to the type of preparation used.

Endoscopic evaluation, however, will be performed by an experienced endoscopist who will be blinded to the type of bowel preparation administered to the patient.

The bowel preparation product used consists of a very low-volume solution of Macrogol (3350) + Ascorbic Acid (1 liter) (PLENVU®, Norgine).

Bowel cleanliness level assessed using the Boston Bowel Preparation Scale adn overall tolerability of bowel preparation are primary outcomes.

Patient compliance and safety; the number of detected lesions, the percentage of complete colonoscopies, the duration of the colonoscopy, the degree of discomfort/pain perceived by the patient during the examination, and procedure-related complications are secondary outcomes.

Patients will be assigned to the two treatment groups according to a computer-generated sequence.

Data will be expressed as mean and standard deviation for quantitative variables and as frequencies for qualitative variables. The comparison between the two groups will be performed using the t-test for quantitative variables and the chi-square (χ²) test for qualitative variables.

Inclusion Criteria:

• All patients scheduled as outpatient for screening or surveillance colonoscopy

Exclusion Criteria:

• Inpatients
• refusal of split dose regimen for bowel preparation
• previous history of colorectal resection
• severe cardiac disease
• advanced (stage IV and V) chronic kidney disease
• pregnancy; ileus
• suspected bowel obstruction or toxic megacolon
• known inflammatory bowel disease
• known or suspected allergy to PEG.