01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Colonoscopy combines whole-colon mucosal inspection with biopsy and polypectomy, but sedation, bowel preparation, perforation and bleeding risks must be balanced against benefit. Flexible sigmoidoscopy may answer a distal question with less preparation, while its limited reach makes it unsuitable for excluding proximal pathology.
CT colonography can assess the whole colon without sedation and also depicts extracolonic structures. It still requires preparation and colonic insufflation, exposes the patient to radiation, and sends suspicious mucosal findings onward to endoscopy for tissue or treatment. Test reports must state preparation quality and anatomical completion.
Key points
- Choose the test by anatomical reach, need for biopsy or therapy, patient fitness and preparation feasibility.
- Colonoscopy examines mucosa throughout the colon and permits biopsy or polypectomy when complete and well prepared.
- Flexible sigmoidoscopy assesses distal bowel only and cannot exclude right-sided disease.
- CT colonography avoids sedation and shows the whole colon, but uses radiation and cannot provide histology.
- Reconcile antithrombotics and comorbidity before an intervention likely to include polypectomy.
- Poor preparation or an unreached caecum leaves an incomplete study that needs a completion plan.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Colonoscopy examines the colonic mucosa and can biopsy or remove lesions, but completion, bowel preparation and withdrawal inspection determine sensitivity.
Flexible sigmoidoscopy reaches the distal colon with less preparation and may answer a focused rectal or left-sided question, yet cannot exclude proximal disease.
CT colonography evaluates the distended colon and extracolonic structures without sedation, but exposes the patient to ionising radiation and cannot provide histology.
Anticoagulant and antiplatelet use, implanted devices, comorbidity and intended polypectomy affect procedural planning and should be reviewed before colonoscopy.
An incomplete or poorly prepared study needs an explicit completion plan because an unseen segment remains diagnostically unresolved.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
ColonoscopyFirst step - Why
- directly inspect the whole colonic mucosa and obtain biopsy or provide endoscopic therapy
- Interpretation and limitations
- Quality depends on caecal intubation, bowel cleanliness and careful withdrawal; perforation and bleeding are uncommon but material risks.
- 02
Flexible sigmoidoscopy - Why
- inspect rectum and left colon for distal disease
- Interpretation and limitations
- A normal distal study cannot exclude right-sided cancer or proximal inflammatory disease when the clinical question covers the whole colon.
- 03
CT colonography - Why
- evaluate the entire colon when colonoscopy is unsuitable, incomplete or selected by the pathway
- Interpretation and limitations
- Suspicious findings usually require subsequent endoscopy for biopsy, and flat lesions may be less conspicuous.
- 04
Histopathology from endoscopic samples - Why
- establish the tissue diagnosis and grade dysplasia or malignancy
- Interpretation and limitations
- Sampling error is possible; radiological and endoscopic discordance should be reviewed rather than resolved by assumption.
- 05
Procedure quality record - Why
- confirm preparation quality, extent, landmarks, lesions, biopsies and complications
- Interpretation and limitations
- A report saying normal is insufficient when the caecum was not reached or preparation was inadequate in the segment of interest.
04Clinical next stepsHow the result changes management or prompts escalation.
01Worked case: applied colonoscopy, flexible sigmoidoscopy and ct colonographyReach a specific decision and confirm it happenedFirst stepA frail 79-year-old with iron-deficiency anaemia needs assessment of the whole colon. Previous sedation caused respiratory compromise, so the team must compare CT colonography with carefully planned colonoscopy and explain downstream biopsy needs.+
- 1Define that the diagnostic target is the entire colon because iron-deficiency anaemia can arise from proximal disease; a flexible sigmoidoscopy would leave that target incompletely assessed.
- 2Review prior respiratory compromise, current fitness, renal function, bowel-preparation safety and whether likely lesions would require immediate biopsy or polypectomy.
- 3Discuss CT colonography as a whole-colon option without sedation, including radiation, extracolonic findings and the possibility of later colonoscopy for tissue.
- 4Agree CT colonography because current procedural risk outweighs immediate therapeutic benefit, and send a request that states the iron-deficiency indication and prior sedation event.
- 5CT colonography shows a 35 mm annular ascending-colon lesion and no synchronous colonic lesion. Refer to the colorectal multidisciplinary team and obtain anaesthetic-supported targeted colonoscopic biopsies, which confirm adenocarcinoma; verify that histology and staging results are acknowledged in the treatment plan.
02Incomplete-colonoscopy completionDo not call unseen bowel normalColonoscopy for iron-deficiency anaemia stops at the splenic flexure because of fixed looping; preparation is good and no distal lesion is found.+
- 1Record the maximum extent, landmarks, preparation, reason for failure and any biopsies rather than issuing a whole-colon normal result.
- 2Choose repeat expert colonoscopy or CT colonography according to fitness, likelihood of therapy and local expertise; flexible sigmoidoscopy would simply repeat the already seen segment.
- 3Verify that the proximal colon is examined and that any radiological lesion receives endoscopic tissue diagnosis or treatment.
03Post-procedure emergencyAssess perforation or haemorrhage immediatelyFour hours after difficult polypectomy, a patient develops severe generalised pain, guarding, pulse 126/min and blood pressure 88/54 mmHg.+
- 1Call emergency, endoscopy, colorectal and anaesthetic teams; begin ABCDE assessment, keep nil by mouth, obtain access, lactate, full blood count, coagulation, renal profile and crossmatch.
- 2Give intravenous fluids, antibiotics for suspected perforation and blood support when bleeding physiology requires it; do not send an unstable patient for an unescorted routine scan.
- 3Obtain urgent contrast CT when safe to define free gas, leak and haemorrhage, then decide radiological, endoscopic or operative source control from the result and clinical trajectory.
- 4Verify falling lactate, stable haemoglobin, abdominal findings and source control in a monitored setting.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Confirm that histology from every relevant biopsy or polyp is reviewed and communicated with a clear management owner.
- After incomplete colonoscopy, specify whether repeat expert colonoscopy or CT colonography will complete the colonic assessment.
- Ask promptly about severe pain, persistent bleeding, fever or collapse after an invasive procedure and assess for perforation or haemorrhage.
- Record recommended surveillance only after integrating polyp number, size, histology, completeness and current surveillance guidance.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
The clinical question chooses the test
A test is appropriate when its reach and capabilities match the decision; a familiar procedure can still be the wrong investigation.
Completion is a result
Failure to visualise the intended segment is clinically meaningful and should trigger a documented alternative rather than a falsely reassuring label.
Histology changes the pathway
Imaging can show a mass and endoscopy can describe it, but tissue diagnosis usually directs cancer treatment and molecular testing.
Preparation is diagnostic quality
Poor cleansing can hide flat lesions and small polyps, so bowel-preparation quality belongs in interpretation rather than administrative notes.
07Common pitfallsFrequent interpretation and management errors.
- 01
Using flexible sigmoidoscopy to exclude proximal pathology answers a larger question than the procedure can support.
- 02
Treating CT colonography as therapeutic overlooks that a positive polyp or cancer finding still needs endoscopic or surgical management.
- 03
Failing to reconcile antithrombotic medicines before planned polypectomy creates preventable bleeding or thrombosis risk.
- 04
Accepting a normal report without checking preparation and examination extent can conceal an incomplete investigation.