Flexible Sigmoidoscopy

Flexible sigmoidoscopy uses a longer, flexible version of the instruments covered in Anoscopy and proctoscopy to examine the rectum and the lower portion of the colon (the sigmoid and descending colon) — reaching further than an anoscope or proctoscope, but not the entire colon the way a colonoscopy does.

Key takeaways

Flexible sigmoidoscopy examines roughly the last third to half of the colon, not the full length.
It generally requires less preparation than a colonoscopy, and sedation is often not needed, though this varies by provider.
It has good sensitivity for cancers and polyps in the area it reaches, but it can miss issues further up the colon — a genuine limitation worth understanding.

What it examines

Flexible sigmoidoscopy uses a flexible, lit tube with a camera, inserted through the anus and advanced through the rectum and into the sigmoid and descending colon — the portion closer to the rectum. It doesn’t reach the transverse or ascending colon further up, which is the fundamental difference from a full colonoscopy.

Why this limitation matters

Research has found that sigmoidoscopy reduces the incidence of cancer in the part of the colon it reaches (the distal colorectum) but doesn’t reduce incidence in the proximal colon — the part it can’t see. This is a meaningful, well-documented gap: a clean flexible sigmoidoscopy rules out problems in the area examined, but doesn’t rule out something further up the colon. For this reason, guidelines generally reserve it as a full replacement for colonoscopy only in specific screening contexts (often paired with an annual stool-based test), and it’s not necessarily sufficient on its own to fully investigate certain symptom patterns or higher-risk situations, where a full colonoscopy is preferred.

What it involves

Preparation is typically lighter than for a full colonoscopy — often an enema shortly before the procedure rather than a full oral bowel prep, though this varies by provider and situation. Sedation is often not required, since the exam is shorter and less extensive, though some patients and providers still opt for it.

When it’s used

As one option within a colorectal cancer screening program, typically paired with an annual stool-based test (FIT) to help cover the portion of the colon it doesn’t reach
To evaluate symptoms specifically localized to the lower colon or rectum
In some healthcare settings, as a faster, lower-resource alternative for initial evaluation, sometimes followed by a full colonoscopy if something concerning is found

What it can’t tell you

A normal flexible sigmoidoscopy doesn’t rule out a problem in the proximal colon. If your symptoms, risk factors, or a family history suggest that region could be relevant, a full colonoscopy is generally the more appropriate test — discuss this directly with whoever is recommending your evaluation rather than assuming one test is equivalent to the other.

Want to understand the full-colon alternative? See When you need a colonoscopy.

This page is for general education. Whether flexible sigmoidoscopy is sufficient for your situation, or whether a full colonoscopy is needed, depends on your specific symptoms and risk factors.