A 52-year-old man from Mumbai undergoes screening for colorectal cancer using a new faecal immunochemical test (FIT). The test has a sensitivity of 92% and specificity of 87%. The pretest probability of colorectal cancer in this asymptomatic screening population is 3%. The patient tests positive. What is the most appropriate next step in management?

Explanation

## Clinical Context This question tests the application of screening test results in clinical decision-making — a high-yield concept in NEET PG biostatistics and preventive medicine. ## Understanding the Positive FIT Result **Key Point:** A positive faecal immunochemical test (FIT) in a colorectal cancer screening programme is an indication for colonoscopy — the gold standard for definitive diagnosis. This is the established clinical pathway per national and international guidelines (USPSTF, Indian guidelines for colorectal cancer screening). ## Calculating Positive Predictive Value (for context) Using Bayes' theorem with Sensitivity = 0.92, Specificity = 0.87, Pretest probability = 0.03: $$PPV = \frac{0.92 \times 0.03}{(0.92 \times 0.03) + (0.13 \times 0.97)} = \frac{0.0276}{0.1537} ≈ 18\%$$ **High-Yield:** Even though the PPV is only ~18%, this represents a 6-fold increase over the baseline 3% pretest probability. The positive likelihood ratio (LR+) = 0.92 / 0.13 ≈ 7.1, which is moderately strong evidence for disease. ## Why Colonoscopy is the Most Appropriate Next Step In the "best next step in management" framework: 1. **Colonoscopy is the established next step** after a positive FIT in any screening programme — this is guideline-driven clinical action (Harrison's Principles of Internal Medicine, 21st ed.) 2. Calculating PPV is a background statistical exercise that informs counselling, but it is NOT itself a "management step" — it does not change the clinical pathway 3. A positive FIT mandates colonoscopy regardless of the exact PPV calculation, because the post-test probability (18%) is substantially above the pre-test probability (3%) 4. Reassurance and repeat FIT (Option A) would be inappropriate — a positive result must be investigated 5. Referral for chemotherapy (Option D) is premature without histological diagnosis ## Why Option C is Incorrect as the "Best Next Step" **Clinical Pearl:** While counselling on PPV is good practice and should accompany the referral, it is not the *most appropriate next step in management*. The actionable clinical decision is to proceed to colonoscopy. Framing "calculate PPV" as the primary next step conflates a statistical exercise with a management action. ## Management Algorithm | Step | Action | |------|--------| | 1 | Positive FIT result | | 2 | Counsel patient (including PPV context) | | **3** | **Proceed to colonoscopy** ← Most appropriate next step | | 4 | Definitive diagnosis ± treatment | **Key Point:** Per Harrison's and standard screening guidelines, a positive FIT → colonoscopy is the correct clinical pathway. The "best next step" is colonoscopy (Option B).