Colonoscopy: How the procedure works step by step

What Is a Colonoscopy and Why Is It Performed

Colonoscopy is a digestive endoscopy procedure that allows direct visualization of the mucosa of the entire colon and, when indicated, the terminal ileum, using a flexible instrument equipped with a video camera (colonoscope).

Main clinical indications include:

• Colorectal cancer screening, generally starting at age 45–50 or following a positive first-line screening test (e.g., fecal occult blood test);
• Evaluation of symptoms such as rectal bleeding, persistent changes in bowel habits, abdominal pain, or unexplained iron-deficiency anemia;
• Post-polypectomy surveillance or monitoring in patients with inflammatory bowel disease (IBD);
• Oncological follow-up after treatment for colorectal cancer.

Colonoscopy allows not only diagnosis but also therapeutic intervention during the same session, such as polypectomy or biopsy sampling, reducing the need for additional procedures.

How a Colonoscopy Works Step by Step

1. Bowel Preparation

Adequate bowel preparation is essential for a high-quality colonoscopy. ESGE (European Society of Gastrointestinal Endoscopy) guidelines recommend split-dose regimens, dividing the laxative solution (typically polyethylene glycol, PEG based) into two administrations: one the evening before the procedure and one in the hours preceding it.

Inadequate bowel cleansing is associated with longer procedure times, a lower adenoma detection rate (ADR), and, in some cases, the need to repeat the examination within a year. For this reason, patients receive precise dietary and dosing instructions in the days leading up to the procedure.

2. Patient Admission and Pre-Procedural Assessment

On the day of the examination, the patient is admitted to the digestive endoscopy unit, where staff collect the medical history, review current medications (particularly anticoagulants and antiplatelet agents), and obtain informed consent. Baseline vital signs (blood pressure, oxygen saturation, heart rate) are also monitored.

3. Sedation and Analgesia

In most centers, colonoscopy is performed under conscious or deep sedation to ensure patient comfort and facilitate scope advancement. The choice of sedation protocol depends on the patient’s clinical status and the center’s policies, in line with recommendations from gastroenterology and anesthesiology scientific societies.

4. Performing the Procedure

The patient is positioned in the left lateral decubitus position. The colonoscope is inserted through the anus and advanced, under direct visual guidance, to the cecum (and occasionally the terminal ileum), using controlled insufflation of air or carbon dioxide (CO2) — the latter preferred because it is reabsorbed more quickly and associated with less post-procedural abdominal distension.

During scope withdrawal, the endoscopist systematically inspects the colonic mucosa, assessing for polypoid lesions, inflammatory changes, or other abnormalities. When indicated, this phase may include:

• Targeted biopsies for histological examination;
• Endoscopic polypectomy using a diathermic snare or biopsy forceps, depending on lesion size and morphology;
• Endoscopic hemostasis, in case of post-polypectomy bleeding.

Withdrawal time is a recognized quality indicator, as excessively short times correlate with reduced diagnostic yield.

5. Recovery and Discharge

At the end of the procedure, the patient is transferred to a recovery area for post-sedation monitoring. Findings are typically discussed with the patient upon waking, while histology results from any biopsies take longer to become available. Patients are then given instructions on resuming diet, managing home medications, and follow-up appointments.

Risks and Complications

Colonoscopy is a safe procedure when performed by qualified personnel, but like any invasive examination it carries rare risks, including perforation, post-polypectomy bleeding, and adverse reactions to sedation. The incidence of major complications is generally below 1% and is further reduced through standardized protocols and appropriate medical devices.