A screening programme for colorectal cancer in a population of 10,000 asymptomatic adults uses faecal occult blood test (FOBT) as the initial screening test. The test has a sensitivity of 90% and specificity of 95%. Among those who test positive, which investigation should be performed next to confirm the diagnosis and determine the location and extent of any lesion?

## Screening vs. Diagnostic Testing Framework This question illustrates the critical principle of **positive predictive value (PPV)** in screening programmes. FOBT is a screening test with high sensitivity (90%) but moderate specificity (95%). A positive FOBT has a PPV of approximately 5–10% in asymptomatic populations (depending on disease prevalence), meaning most positive tests are false positives. ## Why Colonoscopy Is the Investigation of Choice **Key Point:** Colonoscopy is the gold standard confirmatory investigation for colorectal cancer screening. It offers: - Direct visualization of the entire colon and rectum - Ability to obtain tissue diagnosis (biopsy) from suspicious lesions - Therapeutic capability (polypectomy of adenomas) - Sensitivity and specificity >95% for detecting colorectal cancer **High-Yield:** In a positive screening test (FOBT+), colonoscopy is the next investigation because: 1. It confirms or refutes the diagnosis 2. It identifies the exact location and extent of lesions 3. It allows tissue diagnosis and staging 4. It enables therapeutic intervention (polyp removal) **Clinical Pearl:** The positive FOBT has a relatively low PPV (~5–10%) in asymptomatic populations. Colonoscopy is essential to avoid unnecessary anxiety and overtreatment in the majority of FOBT-positive patients who do not have cancer. ## Diagnostic Accuracy of Investigations | Investigation | Sensitivity | Specificity | Tissue Diagnosis | Therapeutic Capability | Cost | | --- | --- | --- | --- | --- | --- | | **Colonoscopy** | 95–98% | 95–99% | Yes (biopsy) | Yes (polypectomy) | Moderate | | CT colonography | 90–95% | 85–90% | No | No | High | | Barium enema | 85–90% | 90–95% | No | No | Low | | Repeat FOBT | 90% | 95% | No | No | Very low | **Mnemonic:** **GOLD** = Gold standard for colorectal cancer diagnosis: - **G**old = Colonoscopy (gold standard) - **O**bservation = Direct visualization - **L**esion = Localization and characterization - **D**iagnosis = Tissue confirmation via biopsy ## Likelihood Ratio Concept A positive FOBT (sensitivity 90%, specificity 95%) has a positive likelihood ratio of: $$LR^+ = \frac{\text{Sensitivity}}{1 - \text{Specificity}} = \frac{0.90}{1 - 0.95} = \frac{0.90}{0.05} = 18$$ This means a positive FOBT is 18 times more likely in patients with colorectal cancer than in those without. However, in a low-prevalence screening population, the PPV remains modest (~5–10%), necessitating confirmatory colonoscopy.