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Difficulty: EasyDiagnostic and Screening Test Performance

A public health department is evaluating a newly developed screening test for a viral infection. The test has a sensitivity of 95% and a specificity of 90%. Initially, the test is deployed in Region A, where the prevalence of the viral infection is 20%. Six months later, the department expands testing to Region B, where the prevalence of the viral infection is only 2%. Assuming the test is performed exactly the same way in both regions, which of the following test parameters will be lower in Region B compared to Region A?

  1. Positive predictive valueAnswer
  2. B
    Negative predictive value
  3. C
    Sensitivity
  4. D
    Specificity
  5. E
    Positive likelihood ratio

Answer

Positive predictive value
The positive predictive value (PPV) is the probability that a patient with a positive test result truly has the disease. PPV is highly dependent on the prevalence of the disease in the tested population. In a population with low prevalence (like Region B at 2%), the absolute number of false-positive results will greatly outnumber the true-positive results. Consequently, the PPV will be significantly lower in Region B than in Region A.

Step-by-Step Solution

1
Identify the key difference between the two populations being tested.
Region B has a much lower disease prevalence (2%) than Region A (20%).
Understanding the baseline differences is necessary to determine which statistical parameters will shift.
2
Evaluate how prevalence affects different diagnostic test parameters.
Sensitivity and specificity are intrinsic to the test and remain unchanged. Negative predictive value (NPV) increases as prevalence decreases. Positive predictive value (PPV) decreases as prevalence decreases.
Applying the mathematical relationships between disease prevalence and predictive values reveals the correct answer.

Key Concept

Positive predictive value (PPV) is directly proportional to disease prevalence in a population, whereas sensitivity, specificity, and likelihood ratios are intrinsic properties of the test that do not change with prevalence.
Estimated Time:45s
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Topics

Diagnostic and Screening Test Performance

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