A 52-year-old man with a family history of colorectal cancer undergoes screening. A faecal occult blood test (FOBT) is performed, which tests positive. The test has a sensitivity of 90% and specificity of 95% in the population. The prevalence of colorectal cancer in this screening population is 2%. Which investigation is most appropriate as the next step to confirm the diagnosis?

Explanation

## Clinical Context A positive FOBT in a screening setting requires definitive confirmation. Although FOBT has reasonable sensitivity (90%) and specificity (95%), the positive predictive value (PPV) in a low-prevalence population (2%) is only ~27%. This means most positive tests are false positives. ## Calculating PPV $$PPV = \frac{TP}{TP + FP} = \frac{Sensitivity \times Prevalence}{(Sensitivity \times Prevalence) + [(1 - Specificity) \times (1 - Prevalence)]}$$ $$PPV = \frac{0.90 \times 0.02}{(0.90 \times 0.02) + (0.05 \times 0.98)} = \frac{0.018}{0.018 + 0.049} \approx 0.27 \text{ or } 27\%$$ **Key Point:** A positive screening test with low PPV requires a high-sensitivity, high-specificity confirmatory test — not another screening test. ## Why Colonoscopy? | Feature | Colonoscopy | CT Colonography | Barium Enema | |---------|-------------|-----------------|---------------| | **Sensitivity** | 95–98% | 90–95% | 85–90% | | **Specificity** | 98–99% | 95–98% | 90–95% | | **Gold Standard?** | Yes | No | No | | **Therapeutic capability** | Yes (biopsy, polypectomy) | No | No | | **Invasiveness** | Invasive | Non-invasive | Non-invasive | **High-Yield:** Colonoscopy is the gold standard for colorectal cancer diagnosis and is the investigation of choice after a positive FOBT because it: 1. Has the highest sensitivity and specificity 2. Allows direct visualization and tissue sampling (biopsy) 3. Enables therapeutic intervention (polypectomy) 4. Is the reference standard against which all other tests are validated **Clinical Pearl:** In low-prevalence screening populations, even a positive test on a reasonably specific test has a modest PPV. Colonoscopy's high sensitivity and specificity ensure accurate confirmation and rule out false positives. [cite:Park 26e Ch 10]