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Difficulty: MediumDiagnostic and Screening Test Performance

A large healthcare network is evaluating the performance of a fecal immunochemical test (FIT) for colorectal cancer screening. Epidemiological data indicates that the prevalence of undetected colorectal neoplasia is 5%5\% in patients aged 50 to 75 years and 1%1\% in patients aged 45 to 49 years. The FIT has a sensitivity of 80%80\% and a specificity of 90%90\%, and these intrinsic test characteristics remain constant across both age groups. If the screening program is expanded from the older age group to include the 45 to 49 year old cohort, which of the following accurately describes the expected predictive values of the FIT in the younger cohort compared to the older cohort?

  1. Decreased positive predictive value and increased negative predictive valueAnswer
  2. B
    Increased positive predictive value and decreased negative predictive value
  3. C
    Decreased positive predictive value and decreased negative predictive value
  4. D
    No change in positive predictive value, but increased negative predictive value
  5. E
    No change in either positive predictive value or negative predictive value

Answer

Decreased positive predictive value and increased negative predictive value
The correct answer accurately reflects the mathematical relationship between disease prevalence and predictive values. Positive predictive value (PPV) is directly proportional to prevalence. When testing a population with a lower prevalence (1%1\% vs 5%5\%), a higher proportion of the positive results will be false positives, thereby decreasing the PPV. Conversely, negative predictive value (NPV) is inversely proportional to prevalence. In a population with lower prevalence, a higher proportion of negative results will be true negatives, thereby increasing the NPV. Sensitivity and specificity remain unchanged because they are intrinsic properties of the FIT.

Step-by-Step Solution

1
Identify the change in disease prevalence between the two groups.
The prevalence of colorectal cancer decreases from 5%5\% (older cohort) to 1%1\% (younger cohort).
Understanding the direction of the prevalence shift is required to determine the impact on predictive values.
2
Determine the relationship between prevalence and positive predictive value (PPV).
As prevalence decreases, the proportion of false positives out of all positive test results increases, leading to a decreased PPV.
PPV depends on the underlying disease prevalence; it is directly proportional to it.
3
Determine the relationship between prevalence and negative predictive value (NPV).
As prevalence decreases, the proportion of true negatives out of all negative test results increases, leading to an increased NPV.
NPV depends on the underlying disease prevalence; it is inversely proportional to it.

Key Concept

The effect of disease prevalence on a diagnostic test's Positive Predictive Value (PPV) and Negative Predictive Value (NPV).
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Topics

Diagnostic and Screening Test Performance

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