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Difficulty: MediumDiagnostic and Screening Test Performance

A clinical investigator is evaluating a newly developed plasma tau biomarker as a screening tool for early-stage Alzheimer disease. The test's performance is initially validated in a specialized memory disorder clinic where the prevalence of the disease is 30%30\%. In this setting, the assay demonstrates a sensitivity of 85%85\% and a specificity of 90%90\%. The investigator plans to implement the same biomarker test in a general primary care setting, where the prevalence of early-stage Alzheimer disease is estimated to be 2%2\%. Assuming the cutoff value for a positive test remains the same, which of the following is the most likely expected change in the test's predictive values in the primary care setting compared to the specialized clinic?

  1. A
    Increased positive predictive value and decreased negative predictive value
  2. B
    Increased positive predictive value and increased negative predictive value
  3. Decreased positive predictive value and increased negative predictive valueAnswer
  4. D
    Decreased positive predictive value and decreased negative predictive value
  5. E
    Predictive values remain unchanged

Answer

Decreased positive predictive value and increased negative predictive value
The correct answer accurately reflects the mathematical relationship between disease prevalence and a test's predictive values. Positive Predictive Value (PPV) is the probability that a patient with a positive test actually has the disease, while Negative Predictive Value (NPV) is the probability that a patient with a negative test truly does not have the disease. When a test is moved to a population with a lower disease prevalence (from 30%30\% to 2%2\%), the number of true positives drops significantly, making false positives proportionally more common among all positive results. Thus, the PPV decreases. Conversely, the number of true negatives increases, making false negatives proportionally less common among all negative results, which increases the NPV.

Step-by-Step Solution

1
Identify the change in disease prevalence between the two settings.
The prevalence decreases from 30%30\% in the memory clinic to 2%2\% in the primary care setting.
Understanding the shift in population parameters is required to evaluate changes in predictive values.
2
Determine the effect of a decreased disease prevalence on the positive predictive value (PPV).
The PPV will decrease.
When a disease is rare, there are fewer true positives and relatively more false positives. Consequently, a positive test result is less likely to represent true disease.
3
Determine the effect of a decreased disease prevalence on the negative predictive value (NPV).
The NPV will increase.
In a population with a very low disease prevalence, the vast majority of patients are healthy. Therefore, true negatives vastly outnumber false negatives, making a negative test result highly reliable.

Key Concept

Effect of disease prevalence on the predictive values of a diagnostic test
Estimated Time:1m 0s
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Topics

Diagnostic and Screening Test Performance

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