A 50-year-old man with no symptoms undergoes screening for colorectal cancer. His fecal occult blood test (FOBT) is positive. Which investigation is the most appropriate next step to confirm the diagnosis and identify the source of bleeding?

Explanation

## Colorectal Cancer Screening — Confirmatory Investigation **Key Point:** Colonoscopy with biopsy is the gold standard confirmatory investigation for a positive FOBT and allows direct visualization, tissue diagnosis, and therapeutic intervention (polypectomy). ### Screening and Diagnostic Algorithm for Colorectal Cancer ```mermaid flowchart TD A[Asymptomatic man age 50+]:::outcome --> B[FOBT/FIT screening]:::action B --> C{FOBT result}:::decision C -->|Negative| D[Repeat in 1 year]:::action C -->|Positive| E[Colonoscopy + biopsy]:::action E --> F{Finding}:::decision F -->|Normal| G[Repeat colonoscopy in 10 years]:::action F -->|Polyp| H[Polypectomy + histology]:::action F -->|Cancer| I[Staging + treatment]:::outcome H --> J{Histology grade}:::decision J -->|Adenoma| K[Surveillance colonoscopy]:::action ``` ### Why Colonoscopy is the Answer **High-Yield:** Colonoscopy offers: 1. **Direct visualization** of the entire colon and rectum 2. **Tissue diagnosis** via biopsy — essential to confirm malignancy 3. **Therapeutic capability** — polypectomy of adenomas, hemostasis 4. **Sensitivity 95%+** for detecting colorectal cancer and polyps 5. **Gold standard** for confirmation of positive FOBT/FIT ### Comparison of Investigations for Positive FOBT | Investigation | Sensitivity | Specificity | Biopsy Capability | Therapeutic Capability | Role | |---|---|---|---|---|---| | **Colonoscopy** | 95%+ | 98% | **Yes** | **Yes (polypectomy)** | **Gold standard confirmatory** | | Barium enema | 85–90% | 90% | No | No | Historical; largely replaced | | CT colonography | 90–95% | 95% | No | No | Alternative if colonoscopy contraindicated | | Repeat FOBT | Variable | Variable | No | No | Increases false negatives; not recommended | **Clinical Pearl:** A positive FOBT has a positive predictive value of ~10–15% for colorectal cancer and ~30–40% for any colorectal pathology (polyps, cancer, diverticulosis). Colonoscopy is mandatory to identify the source. **Warning:** Repeating FOBT after a positive result delays diagnosis and increases the risk of missing early-stage cancer. This is a common trap in screening questions. **Mnemonic:** **FOBT-Positive → Colonoscopy** (FPC rule) — always proceed to colonoscopy for tissue diagnosis and therapeutic intervention. [cite:Park 26e Ch 7; Harrison 21e Ch 297]