Written and maintained by CASRAI Editorial Board
Last updated
General information for lab, clinic and supply staff. It is not medical advice and does not describe how to perform a procedure; questions about a test belong with a clinician.
Short answer
A sigmoidoscope is the endoscope used to look inside the rectum and lower colon. The NIDDK describes it as a tube with a camera on one end, also called a “scope”. It comes in two forms: a flexible scope that reaches the rectum, sigmoid colon and descending colon, and a shorter rigid scope that examines only the rectum. In US device regulation it falls under the generic endoscope type in 21 CFR 876.1500.
Definition and scope
The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) defines flexible sigmoidoscopy as a procedure in which a doctor uses a flexible tube with a camera on one end, called a sigmoidoscope or scope, to look inside the rectum and lower colon. The lower colon includes the sigmoid colon and the descending colon (NIDDK, “Flexible Sigmoidoscopy”, checked 2026-10-30).
The name therefore refers to the instrument, while “sigmoidoscopy” refers to the examination performed with it. Clinic pages usually describe the examination; this page looks at the instrument and the equipment around it.
What the instrument does
According to the NIDDK, the camera at the end of the scope sends a video image to a monitor, which lets the doctor look at the lining of the lower colon and rectum. This page does not describe how the examination is carried out; questions about preparation, risks and follow-up are for the treating clinician. For the specialty that uses these instruments, see the CASRAI guide to gastroenterology.
Flexible vs rigid sigmoidoscope
Cleveland Clinic (page last updated 2026-04-23, checked 2026-10-30) separates the two forms by what they can examine.
| Feature | Flexible sigmoidoscope | Rigid sigmoidoscope |
|---|---|---|
| Tube | Thin, bendable tube with a light and camera on the end (Cleveland Clinic) | Shorter, straight tube (Cleveland Clinic) |
| Area examined | Rectum and lower colon, including the sigmoid and descending colon (Cleveland Clinic; NIDDK) | Rectum only (Cleveland Clinic) |
| Frequency of use | Not rated here | Used less commonly today (Cleveland Clinic) |
Flexible sigmoidoscope
This is the instrument both the NIDDK and Cleveland Clinic describe on their sigmoidoscopy pages. Its reach covers the rectum, sigmoid colon and descending colon.
Rigid sigmoidoscope
Cleveland Clinic describes it as a shorter, straight tube that examines only the rectum, and notes that providers use it less commonly today.
Is a flexible sigmoidoscopy the same as a colonoscopy?
No. Both use a flexible tube with a camera, but they cover different amounts of bowel. The NIDDK says colonoscopy shows the entire colon, while flexible sigmoidoscopy looks at the rectum and lower colon. Cleveland Clinic states that flexible sigmoidoscopy is shorter and examines less of the colon than a colonoscopy.
For equipment purposes the two instruments are separate scopes, although 21 CFR 876.1500 lists both colonoscopes and sigmoidoscopes as examples of the same generic endoscope type.
What is the difference between a proctosigmoidoscopy and a sigmoidoscopy?
The NIDDK and Cleveland Clinic pages cited here do not define “proctosigmoidoscopy” as a separate procedure, so this page does not assign it a distinct meaning. What can be stated is how the terms map to instruments. 21 CFR 876.1500 lists proctoscopes and sigmoidoscopes as separate named examples of endoscopes, and Cleveland Clinic ties the reach of the instrument to its design: a rigid scope examines the rectum only, a flexible scope also reaches the sigmoid and descending colon.
When a service, order or product listing uses either word, the practical questions are which part of the bowel the scope is designed to reach and whether it is rigid or flexible. The labelling or instructions for use of the specific device answer that.
Where it matters for labs, clinics and supply decisions
In the United States, FDA regulates the instrument under 21 CFR 876.1500, “Endoscope and accessories” (checked 2026-10-30 at the Cornell Legal Information Institute copy of the eCFR). The identification describes a device used to provide access, illumination and allow observation or manipulation of body cavities, consisting of various rigid or flexible instruments, and names sigmoidoscopes among its examples. The regulation also sorts the surrounding items by class, which helps separate the scope from its accessories.
| Item named in 21 CFR 876.1500(b) | Classification stated |
|---|---|
| Endoscope disinfectant basin; single-use endoscopic magnetic retriever; LED light source; endoscope transformer | Class II (special controls), exempt from premarket notification subject to the limitations in 21 CFR 876.9 |
| Photographic accessories; inflation bulb; bite block; cleaning brush for endoscope | Class I, exempt from premarket notification subject to the limitations in 21 CFR 876.9 |
The same regulation lists cleaning accessories, photographic accessories, fiberoptic illuminators and endoscope lamps alongside the scopes, which shows how many separate components can sit around one examination room set-up. The regulation text does not list manufacturer products, product codes or prices. For the supporting workflow, see the CASRAI guides on the endoscopy center supply checklist, endoscope reprocessing and the general endoscope overview. Device class and exemption terms are explained in FDA medical device classification.
Common misunderstandings
- “Sigmoidoscope” is not the whole colon scope. The NIDDK limits flexible sigmoidoscopy to the rectum and lower colon; colonoscopy covers the entire colon.
- Rigid and flexible are not interchangeable. Per Cleveland Clinic, the rigid instrument examines the rectum only.
- A sigmoidoscope is not an accessory. Under 21 CFR 876.1500 it is an example of the endoscope itself; bite blocks, cleaning brushes and light sources are listed as separate items with their own classification.
- The regulation page is not a product listing. It identifies a generic device type and says nothing about brands, sizes or availability.
FAQ
What is a sigmoidoscope?
The tube with a camera used to examine the rectum and lower colon, called a sigmoidoscope or scope by the NIDDK. It may be flexible or rigid.
Is a flexible sigmoidoscopy the same as a colonoscopy?
No. A colonoscopy shows the entire colon; a flexible sigmoidoscopy examines the rectum and lower colon only (NIDDK; Cleveland Clinic).
What is the difference between a proctosigmoidoscopy and a sigmoidoscopy?
The NIDDK and Cleveland Clinic pages do not define a separate procedure under that name. Check the device labelling or the clinic for whether the scope is rigid (rectum only) or flexible (rectum, sigmoid and descending colon).
Which part of the bowel does a rigid sigmoidoscope examine?
Cleveland Clinic states that it examines only the rectum.
Sources
- NIDDK, Flexible Sigmoidoscopy, checked 2026-10-30.
- Cleveland Clinic, Flexible Sigmoidoscopy, page last updated 2026-04-23, checked 2026-10-30.
- 21 CFR 876.1500, Endoscope and accessories (eCFR text as reproduced by the Legal Information Institute), checked 2026-10-30.
