An occupational health clinic at a multinational manufacturing corporation routinely screens its employees for latent tuberculosis infection using an interferon-gamma release assay. Historically, the clinic only screened employees relocating from regions with highly endemic tuberculosis, where the estimated disease prevalence is . A new corporate directive mandates that all employees, including those from regions where the prevalence is estimated at , must now undergo identical screening. Assuming the inherent diagnostic accuracy of the assay remains constant, which of the following changes in test performance characteristics is most likely to occur when screening the low-endemic employee population compared to the high-endemic population?
- Decreased positive predictive value and increased negative predictive valueAnswer
- BIncreased positive predictive value and decreased negative predictive value
- CDecreased sensitivity and increased specificity
- DUnchanged positive predictive value and unchanged negative predictive value
- EDecreased positive predictive value and decreased sensitivity
Answer
Decreased positive predictive value and increased negative predictive value
When a diagnostic or screening test is applied to a population with a lower disease prevalence, the absolute number of true positive results decreases while the relative proportion of false positive results increases. Since Positive Predictive Value (PPV) is the ratio of true positives to all positives, a decrease in prevalence leads directly to a decreased PPV. Conversely, because the vast majority of the low-endemic population does not have the disease, the proportion of true negatives increases relative to false negatives, which increases the Negative Predictive Value (NPV).
Step-by-Step Solution
Key Concept
Predictive values (PPV and NPV) are highly dependent on disease prevalence, whereas inherent test characteristics (sensitivity and specificity) are not.