A screening programme for cervical cancer uses two HPV-based tests: Test A (sensitivity 92%, specificity 78%) and Test B (sensitivity 75%, specificity 95%). Which diagnostic property best distinguishes these tests' performance in identifying women who actually have cervical cancer when the test is positive?

Question

Accepted Answer

D. Test B's higher specificity makes it superior for confirming disease. ## Discriminating Feature: Specificity and Positive Predictive Value

**Key Point:** When a test is **positive**, the ability to confirm that disease is truly present depends on **specificity**, not sensitivity. Test B's 95% specificity ensures fewer false positives, yielding a higher PPV — the probability that a positive test result reflects true disease.

### Why Specificity Matters for Confirmation

**High specificity** = few false positives = when test is positive, disease is likely present.

**High sensitivity** = few false negatives = when test is negative, disease is likely absent.

The question asks about identifying women **when the test is positive** — this is the domain of specificity and PPV.

### Comparative Analysis

| Property | Test A (92% Sen, 78% Spec) | Test B (75% Sen, 95% Spec) |
|----------|---------------------------|---------------------------|
| **Sensitivity** | 92% (fewer false negatives) | 75% |
| **Specificity** | 78% (more false positives) | 95% (fewer false positives) |
| **When test is POSITIVE** | ~22% chance of false positive | ~5% chance of false positive |
| **PPV (at 15% prevalence)** | ~52% | ~79% |
| **Clinical role** | Screening (exclude disease) | Confirmation (confirm disease) |

### Mnemonic: **SpPin** — **Sp**ecificity rules **In**

When you want to **confirm** disease (rule in), choose the test with high **Sp**ecificity. Test B's 95% specificity makes it the superior confirmatory test.

**High-Yield:** In a positive test scenario, specificity is the discriminating property. Test B is superior because:
1. Fewer false positives (95% specificity vs 78%)
2. Higher PPV across all prevalence levels
3. More reliable for confirming cervical cancer diagnosis

**Clinical Pearl:** In cervical cancer screening, a positive HPV test must be confirmed by cytology or colposcopy. A high-specificity test (Test B) reduces unnecessary referrals and anxiety from false positives, making it the preferred confirmatory test.

Answer

A. Test A's higher sensitivity makes it superior for confirming disease

Answer

B. Test A's negative predictive value is always higher

Answer

C. Both tests have equivalent positive predictive values regardless of prevalence

Explanation

Discriminating Feature: Specificity and Positive Predictive Value

Why Specificity Matters for Confirmation

Comparative Analysis

Mnemonic: SpPin — Specificity rules In

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Property	Test A (92% Sen, 78% Spec)	Test B (75% Sen, 95% Spec)
Sensitivity	92% (fewer false negatives)	75%
Specificity	78% (more false positives)	95% (fewer false positives)
When test is POSITIVE	~22% chance of false positive	~5% chance of false positive
PPV (at 15% prevalence)	~52%	~79%
Clinical role	Screening (exclude disease)	Confirmation (confirm disease)