A new diagnostic test for active pulmonary TB is evaluated in a tertiary hospital (high disease prevalence = 30%) and a primary care centre (low prevalence = 5%). The test has fixed sensitivity of 85% and specificity of 90%. Where will the Positive Predictive Value (PPV) be HIGHER?

• A At the primary care centre (low prevalence setting)
• B At the tertiary hospital (high prevalence setting) ✓
• C PPV is the same regardless of prevalence as it depends only on sensitivity and specificity
• D PPV is higher where the test is performed by more experienced technicians

Explanation

PPV increases with increasing disease prevalence in the tested population. At 30% prevalence (per 1000): TP = 300 × 0.85 = 255; FP = 700 × 0.10 = 70; PPV = 255/325 ≈ 78%. At 5% prevalence: TP = 50 × 0.85 = 42.5; FP = 950 × 0.10 = 95; PPV = 42.5/137.5 ≈ 31%. PPV is strongly dependent on pre-test probability (prevalence) — this is why a positive test in a high-risk population is more meaningful than in a low-risk population.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.