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Difficulty: EasyDiagnostic and Screening Test Performance

A public health department uses a fecal immunochemical test to screen for colorectal cancer. Initially, the test is administered to the general population aged 5050 to 7575 years, where the prevalence of the disease is relatively low. The department later updates its protocol to use the test exclusively in a high-risk cohort of individuals with a strong family history of colorectal cancer. The prevalence of the disease in this high-risk cohort is significantly higher than in the general population. Assuming the test's sensitivity and specificity remain constant, which of the following best describes how the predictive values of the fecal immunochemical test will change when applied to the high-risk cohort compared to the general population?

  1. Positive predictive value will increase, and negative predictive value will decrease.Answer
  2. B
    Positive predictive value will decrease, and negative predictive value will increase.
  3. C
    Both positive predictive value and negative predictive value will increase.
  4. D
    Both positive predictive value and negative predictive value will decrease.
  5. E
    The predictive values will remain unchanged because they are intrinsic properties of the test.

Answer

Positive predictive value will increase, and negative predictive value will decrease.
The positive predictive value (PPV) is the probability that a patient has the disease given a positive test result, and the negative predictive value (NPV) is the probability that a patient does not have the disease given a negative test result. When a test is applied to a population with a higher disease prevalence (such as moving from the general population to a high-risk cohort), the proportion of true positive results increases relative to false positive results, leading to an increased PPV. Conversely, the higher prevalence also leads to an increased number of false negative results relative to true negative results, which causes the NPV to decrease. The intrinsic properties of the test (sensitivity and specificity) remain unchanged.

Step-by-Step Solution

1
Identify the primary variable changing in the scenario.
The disease prevalence increases from the general population to the high-risk cohort.
Understanding the shifting population parameter is necessary to determine the effect on test metrics.
2
Recall the relationship between disease prevalence and positive predictive value (PPV).
As prevalence increases, PPV increases.
A higher prevalence means a greater proportion of true positives among all positive test results.
3
Recall the relationship between disease prevalence and negative predictive value (NPV).
As prevalence increases, NPV decreases.
A higher prevalence means more false negatives occur among all negative test results, lowering the probability that a negative result represents a true negative.

Key Concept

The positive predictive value (PPV) of a diagnostic test increases and the negative predictive value (NPV) decreases as the prevalence of the disease in the tested population increases.
Estimated Time:1m 0s
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Topics

Diagnostic and Screening Test Performance

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