Regarding positive predictive value (PPV) and negative predictive value (NPV) in diagnostic test evaluation, all of the following statements are correct EXCEPT:

## Concept Overview **Key Point:** PPV and NPV are post-test probabilities that are fundamentally dependent on disease prevalence in the population being tested. Unlike sensitivity and specificity (which are intrinsic test properties), PPV and NPV change with prevalence. ## Analysis of Each Statement ### Statement 1: PPV increases with prevalence — **CORRECT** PPV = $\frac{TP}{TP + FP}$ When prevalence increases, the number of true positives increases relative to false positives, causing PPV to rise. This is a direct mathematical relationship. **Clinical Pearl:** A test with 95% specificity will have much higher PPV in a high-prevalence setting (e.g., screening symptomatic patients) than in a low-prevalence setting (e.g., screening asymptomatic populations). ### Statement 2: NPV is independent of prevalence — **INCORRECT** This is the **trap answer**. NPV is **NOT** independent of prevalence. NPV = $\frac{TN}{TN + FN}$ NPV **depends on**: - Test sensitivity (affects FN) - Test specificity (affects TN) - **Disease prevalence** (affects the ratio of TN to FN) When prevalence decreases, there are more true negatives and fewer false negatives, so NPV increases. NPV is therefore **highly dependent on prevalence**. **High-Yield:** This is a common misconception in exam questions. Students often confuse the independence of sensitivity/specificity from prevalence with the independence of NPV/PPV — they are NOT the same. ### Statement 3: High sensitivity useful for ruling out disease — **CORRECT** A test with high sensitivity has a low false-negative rate. **Mnemonic:** **SnNout** — **Sn**ensitivity rules **N**egative **out**. When a highly sensitive test is negative, disease is ruled out. ### Statement 4: PPV and NPV are post-test probabilities — **CORRECT** Both PPV and NPV are post-test probabilities that incorporate: - Pre-test probability (disease prevalence) - Test sensitivity and specificity ## Summary Table | Parameter | Depends on Prevalence? | Depends on Sensitivity? | Depends on Specificity? | |-----------|------------------------|-------------------------|-------------------------| | Sensitivity | No | — | No | | Specificity | No | No | — | | PPV | **Yes** | Yes | Yes | | NPV | **Yes** | Yes | Yes | [cite:Park 26e Ch 10]