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Difficulty: MediumDiagnostic and Screening Test Performance

An epidemiologic screening program adopts a point-of-care serologic test for a chronic viral infection. The assay operates with a fixed sensitivity of 90% and specificity of 95%. During the first phase, it is utilized exclusively within an urban syringe services program where the target infection is highly prevalent. Six months later, the identical assay is implemented at a suburban university student health center where the infection is exceedingly rare. Which of the following best describes how the predictive values of the test at the university health center will differ from those at the syringe services program?

  1. A
    Higher positive predictive value, lower negative predictive value
  2. B
    Equal positive predictive value, equal negative predictive value
  3. C
    Lower positive predictive value, lower negative predictive value
  4. Lower positive predictive value, higher negative predictive valueAnswer
  5. E
    Higher positive predictive value, higher negative predictive value

Answer

The predictive values at the university health center will demonstrate a lower positive predictive value and a higher negative predictive value.
The correct answer accurately reflects the mathematical relationship between disease prevalence and predictive values. Positive predictive value (PPV) is defined as True Positives / (True Positives + False Positives). As disease prevalence decreases in a population (such as moving to a university health center), the proportion of individuals without the disease increases, leading to a higher absolute number of false positives. This dilutes the true positive results, causing the PPV to fall. Conversely, negative predictive value (NPV) is True Negatives / (True Negatives + False Negatives). With a lower prevalence, there are far more true negatives and fewer false negatives, driving the NPV higher and making a negative result highly reliable.

Step-by-Step Solution

1
Identify the intrinsic versus population-dependent characteristics of the diagnostic test.
Sensitivity and specificity remain constant regardless of the population, whereas positive predictive value (PPV) and negative predictive value (NPV) depend heavily on the pre-test probability (disease prevalence).
To establish which values will change when moving between the two distinct clinical settings.
2
Determine the shift in disease prevalence between the two described populations.
The prevalence drops significantly from the urban syringe services program (high prevalence) to the suburban university student health center (exceedingly rare).
Prevalence is the mathematical driver of changes in predictive values.
3
Analyze the mathematical effect of decreased prevalence on PPV and NPV.
As prevalence decreases, the absolute number of individuals without the disease grows, which increases the proportion of false positives among all positive tests, thereby lowering the PPV. Conversely, true negatives increase relative to false negatives, raising the NPV.
To determine the exact directional change for both predictive values to select the correct statement.

Key Concept

The dependence of positive and negative predictive values on disease prevalence.
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Topics

Diagnostic and Screening Test Performance

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