Currently, colonoscopy is a minimally invasive method that can be used as a diagnostic and therapeutic method by endoscopists, gastroenterologists and coloproctologists. Due to the importance and big impact this method has, it is necessary to both optimize its efficiency, and improve its quality, which is one of the main objectives of this protocol.

By observing which position its faster and which one also results in fewer complications when performing a colonoscopy without reducing its performance and following all the internationally established quality standards regarding colonoscoscopy.

The risk of this protocol implies a risk no greater than the minumum the procedure itself has, and does not generate extra cost for all of the patients subjected to this protocol.

Currently, colonoscopy is one of the most used procedures when it comes to the study and treatment of patients with gastrointestinal conditions, including colorectal cancer, inflammatory bowel disease, etc.

In the area of colorectal neoplasia, colonoscopy has three main functions, which are to diagnose the desease itself and prevent its development by detecting and eliminating potentially premalignant lesions, as well as providing a diagnosis of cancer at an early stage.

The effectiveness of colonoscopy is crucial to carry out an accurate examination of the entire colorectal mucosa, which is why the quality of the procedure has been a subject of study in recent years. Among multiple factors that influence the quality of colonoscopies, the investigators can mention intestinal preparation, which is essential for an accurate procedure, because if patients have an inadequate preparation, it could impair the detection of lesions, since usually, in patients with little or no preparation, colonoscopy can be either incomplete, which requires the study to be repeated, or in case the study continues despite poor intestinal preparation, the presence of feces implies poor visualization of the colonic mucosa, which reduces the ability to detect lesions such as polyps, especially if they are <5 mm. Therefore, the type of solution and tolerability, the preparation regimen and the moment in which the intestinal preparation is performed are considerations to evaluate when performing a colonoscopy.

The position during the colonoscopy is another factor that can influence colonoscopy´s effectiveness, and also, the main focus for the investigators to study. Traditionally, if no position changes occur during colonoscopy, it begins and ends in the left lateral position. However, recent evidence suggests that supine position may reduce the disadvantages of the left lateral position, through decreased frequency of position changes and decreased abdominal pressure, which may result in an easier endoscope insertion in supine position when comparing it to left lateral position, however, there is very few information on the optimal insertion technique in colonoscopy, but it has been observed that in left lateral position, the air leaves the left colon causing it to collapse and also creating sharp curves that can be difficult to overcome during the procedure.

As previously mentioned, colonoscopy insertion is technically challenging, and one of the few clinical trials available that targeted the determination of optimal patient positioning during colonoscopy insertion compared the supine starting position with the left lateral starting position, and the investigators found that cecal intubation times decreased and patient comfort scores improved when using the supine position.

Therefore, patient positioning in colonoscopy has been proposed as a simple and inexpensive technique to increase luminal distension and improve navigation through the colon. Based on what has been mentioned before, using the initial supine position could be a convenient method to reduce cecal intubation time, reduce pain, and improve acceptance of colonoscopy among patients. However, more research is necessary in this area to stablish the advantages of the initial supine position over other positions thar are classically used.

Based on everything that has been described above, the investigators asked themselves the following research question to start this clinical trial: Is there a significant difference in the effectiveness and comfort of performing colonoscopy, when comparing the left lateral position with the supine position?

The institutes where this clinical trial will take place have subjects for study that are ellegible to enroll as participants, infraestructure and health care providers trained to perform colonoscopies and a complete investigators team to collect and analyze the data for this protocol.

The protocol will be limited only to patients who are beneficiaries of each of the hospitals involved. Furthermore, as it is a single-blind randomized clinical trial, the endoscopists in the study cannot be blinded, so investigator bias will not be excluded.

Inclusion Criteria:

• Patients with indication for colonoscopy in our coloproctology service
• Both women and men
• Patients within an age range of 18-79 years
• Patients who agree having a colonoscopy and who sign the informed consent to participate in the protocol

Exclusion Criteria:

• Patients under 18 or over 80 years old
• All patients that won´t like to participate in the protocol or won´t sign the informed consent
• Pregnant women
• Patients with a medical record of colonic resection, ostomy status, severe cardiopulmonary and renal diseases, major psychiatric disorders, therapeutic colonoscopy or any contraindications for colonoscopy
• Non compliance with the colonic preparation regimen
• Active bleeding during the procedure
• Patients with a known diagnosis of colorectal cancer
• Patients with class III obesity

Elimination criteria:

• Boston score <6 for colonic preparation
• Patients with a colonic lesion that makes it difficult to pass the colonoscope
• Patients with insufficient sedation that requires the procedure to stop temporarily
• Bowel perforation during colonoscopy
• Inability to reach the cecum despite loop reduction maneuvers
• Indication to suspend the study given by the anesthesiologist

Eligible patients, according to established guidelines, were scheduled for colonoscopy and assigned a randomized number.

During the colonoscopy (procedure duration: approximately 20-60 minutes per participant).

Of the 144 participants studied, no adverse events or deaths were recorded.

The relatively small sample size per group may have limited the statistical power to detect subtle differences, and the absence of stratified analyses by hospital with respect to clinical variables (such as age and comorbidities) could have influenced both intubation time and procedure tolerance.

The main investigators will be in charge of randomizing all the patients ellegible for this protocol, and the care providers and endoscopists will be masked when it comes to knowing to which arm of the study each patient belongs to, untill the moment they need to perform the colonoscopy.