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Difficulty: MediumDiagnostic Testing Performance

A new self-administered screening questionnaire for generalized anxiety disorder (GAD) is validated in a specialized psychiatric outpatient clinic where the prevalence of GAD is 40%40\%. In this high-prevalence setting, the questionnaire has a sensitivity of 90%90\%, a specificity of 80%80\%, a positive predictive value (PPV) of 75%75\%, and a negative predictive value (NPV) of 92%92\%.

Administrators at a large primary care network decide to use this questionnaire for routine screening in their general medicine clinics. Epidemiological data confirms that the prevalence of GAD in this general practice population is 10%10\%. Assuming the intrinsic test characteristics (sensitivity and specificity) of the screening tool remain identical, which of the following describes the expected change in the predictive values when the questionnaire is used in the primary care clinics rather than the psychiatric clinic?

  1. The positive predictive value will decrease, and the negative predictive value will increase.Answer
  2. B
    The positive predictive value will increase, and the negative predictive value will decrease.
  3. C
    Both the positive predictive value and the negative predictive value will decrease.
  4. D
    Both the positive predictive value and the negative predictive value will increase.
  5. E
    The positive predictive value and the negative predictive value will remain unchanged.

Answer

The positive predictive value will decrease, and the negative predictive value will increase.
The correct answer identifies that lowering the prevalence of a disease within the tested population leads to a decreased positive predictive value (PPV) and an increased negative predictive value (NPV). When a test is applied to a population where the disease is rare (10% vs 40%), the vast majority of people tested are healthy. Even with strong specificity, this large healthy denominator will generate many false positive results, causing the PPV to drop. Conversely, because the disease is rare, a negative result is very reliable, causing the NPV to rise.

Step-by-Step Solution

1
Identify the change in the population parameter.
The disease prevalence is decreasing from 40% in the psychiatric clinic to 10% in the primary care clinics.
Recognizing the direction of prevalence shift is required to determine the impact on predictive values.
2
Determine the effect of decreased prevalence on the positive predictive value (PPV).
The PPV will decrease.
PPV is the probability that a person with a positive test actually has the disease. In a lower prevalence setting, there are far more healthy individuals, leading to a proportionally higher number of false positives relative to true positives.
3
Determine the effect of decreased prevalence on the negative predictive value (NPV).
The NPV will increase.
NPV is the probability that a person with a negative test truly does not have the disease. In a lower prevalence setting, negative tests are highly likely to be true negatives simply because the disease is rare in the general population.

Key Concept

Impact of disease prevalence on predictive values
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Topics

Diagnostic Testing Performance

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