## Negative Predictive Value (NPV) Definition **Key Point:** NPV is the probability that a person with a **negative test result** actually does **not** have the disease. It answers the clinical question: "If my test is negative, how confident can I be that I don't have the disease?" ### Formula $$NPV = \frac{TN}{TN + FN} = \frac{\text{Specificity} \times (1 - \text{Prevalence})}{\text{Specificity} \times (1 - \text{Prevalence}) + (1 - \text{Sensitivity}) \times \text{Prevalence}}$$ ### Clinical Interpretation An NPV of 98% means: - Out of 100 people who test **negative**, approximately 98 truly do not have TB - Only about 2 people with a negative test may actually have TB (false negatives) - This is reassuring for ruling out disease ### Comparison Table: PPV vs NPV | Metric | Definition | Clinical Use | Depends On | |--------|-----------|--------------|------------| | **PPV** | Probability of disease given a **positive** test | Confirms diagnosis | Sensitivity, specificity, prevalence | | **NPV** | Probability of **no disease** given a **negative** test | Excludes diagnosis | Sensitivity, specificity, prevalence | **High-Yield:** A high NPV is essential for screening and rule-out tests (e.g., D-dimer for PE, troponin for MI). A negative result with high NPV provides strong reassurance. **Clinical Pearl:** In TB screening, an NPV of 98% means a negative test is highly reliable for excluding active TB, making it suitable for population screening.
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