A population-based screening program in Delhi aims to detect colorectal cancer in asymptomatic adults aged 50–75 years. Which investigation offers the best balance of sensitivity, specificity, and feasibility for population screening?

Explanation

## Colorectal Cancer Screening: Investigation Selection for Population Programs **Key Point:** For a **population-based** screening program (as opposed to individual clinical screening), Fecal Occult Blood Test (FOBT) offers the best balance of sensitivity, specificity, and **feasibility** — the critical qualifier in the stem. ### Why Feasibility Matters in Population Screening Population-based screening programs must be scalable, affordable, non-invasive, and acceptable to large numbers of asymptomatic individuals. The stem explicitly asks for the "best balance of sensitivity, specificity, **and feasibility**" — this is the key discriminator. ### Comparison of Colorectal Cancer Screening Modalities | Investigation | Sensitivity | Specificity | Interval | Feasibility for Population Program | |---|---|---|---|---| | **FOBT** | 50–70% | 95–98% | Annual | ✅ Non-invasive, cheap, no bowel prep, scalable | | Colonoscopy | 95–99% | 95–99% | 10 years | ❌ Invasive, expensive, requires sedation & bowel prep, limited endoscopist availability | | Flexible sigmoidoscopy | 75–85% | High | 5 years | ⚠️ Semi-invasive, misses proximal lesions | | CT colonography | 90–95% | 90–95% | 5 years | ❌ Radiation, bowel prep, cannot remove polyps, high cost | **High-Yield:** In the context of **public health / PSM**, population-based screening programs (e.g., national cancer screening programs in India, UK NHS Bowel Cancer Screening Programme, US Preventive Services Task Force guidelines) universally recommend **FOBT as the primary population-level screening tool** because: - **Non-invasive**: Stool sample collected at home — no bowel preparation, no sedation - **Low cost**: Highly affordable and scalable in resource-limited settings like India - **High specificity (~95–98%)**: Minimizes false positives and unnecessary follow-up colonoscopies - **Proven mortality reduction**: Randomized controlled trials (Mandel et al., Minnesota Colon Cancer Control Study) demonstrated 33% reduction in colorectal cancer mortality with annual FOBT - **Acceptable to asymptomatic populations**: High uptake rates compared to invasive procedures **Clinical Pearl:** Colonoscopy is the **gold standard for diagnosis and individual clinical screening** but is NOT feasible as a primary population-based screening tool due to resource constraints, invasiveness, and limited endoscopy capacity — especially relevant in the Indian public health context (Delhi population-based program as stated in the stem). **Key Distinction (PSM perspective):** - **Individual/clinical screening** → Colonoscopy every 10 years (gold standard) - **Population-based screening program** → FOBT (best balance of sensitivity, specificity, AND feasibility) **Mnemonic:** **F**OBT = **F**easible, **F**irst-line for **F**ield-level population screening programs. ### Why Other Options Are Less Suitable for Population-Based Programs - **Colonoscopy every 10 years**: Gold standard diagnostically, but invasive, expensive, requires trained endoscopists and infrastructure — not scalable for population-level programs in India. - **CT colonography**: High sensitivity but requires radiation, bowel prep, and cannot remove polyps; positive findings still require colonoscopy; not cost-effective at population level. - **Flexible sigmoidoscopy**: Misses ~50% of proximal colon lesions; semi-invasive; less feasible than FOBT for mass screening. [cite: Park's Textbook of Preventive and Social Medicine, 26e, Ch. 10 — Cancer Screening; WHO Guidelines for Screening and Treatment of Colorectal Cancer]