1. What is PPV and NPV?

Positive Predictive Value (PPV) is the probability that subjects with a positive screening test truly have the disease. Negative Predictive Value (NPV) is the probability that subjects with a negative screening test truly don't have the disease.

2. How Does the Calculator Work?

The calculator uses these equations:

Where:

• \( sensitivity \) — True positive rate (0-1)
• \( specificity \) — True negative rate (0-1)
• \( prevalence \) — Disease prevalence in population (0-1)

Explanation: PPV increases with higher prevalence and specificity, while NPV increases with lower prevalence and higher sensitivity.

3. Importance of PPV/NPV Calculation

Details: These values help clinicians interpret test results in context of disease prevalence, showing how likely a positive or negative test result is correct.

4. Using the Calculator

Tips: Enter sensitivity, specificity, and prevalence as values between 0 and 1 (e.g., 0.95 for 95%). All values must be valid probabilities.

5. Frequently Asked Questions (FAQ)

Q1: Why does PPV change with prevalence?
A: PPV depends on the underlying disease probability. Even with excellent test characteristics, PPV will be low for rare diseases.

Q2: What's the relationship between PPV and specificity?
A: Higher specificity reduces false positives, directly improving PPV.

Q3: When is NPV most useful?
A: NPV is particularly valuable for ruling out disease when the test is negative, especially for tests with high sensitivity.

Q4: Can PPV be calculated without prevalence?
A: No, prevalence is essential for PPV calculation as it represents the pre-test probability.

Q5: How do sensitivity and specificity affect NPV?
A: Higher sensitivity improves NPV by reducing false negatives, while specificity has less direct impact on NPV.