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Difficulty: MediumDiagnostic and Screening Test Performance

A 45-year-old woman is evaluated in the emergency department for right calf pain and swelling. The clinical suspicion for deep vein thrombosis is moderate. The hospital laboratory utilizes a quantitative D-dimer assay as part of the diagnostic algorithm. To reduce the burden on the radiology department and decrease the number of lower extremity Doppler ultrasounds performed for false-positive results, the hospital's medical executive committee decides to raise the threshold (cutoff level) required for a D-dimer test to be reported as "positive." Which of the following describes the most likely impact of this policy change on the diagnostic parameters of the D-dimer test?

  1. Decreased sensitivity and increased positive predictive valueAnswer
  2. B
    Increased sensitivity and decreased negative predictive value
  3. C
    Increased specificity and increased negative predictive value
  4. D
    Decreased specificity and decreased positive predictive value
  5. E
    Decreased sensitivity and increased negative predictive value

Answer

Decreased sensitivity and increased positive predictive value
When a diagnostic test's cutoff threshold is raised, a higher value is needed to classify a result as positive. This strictly reduces the total number of positive test results. Consequently, the test will correctly identify fewer patients who actually have the disease (decreased true positives = decreased sensitivity). At the same time, it will incorrectly flag fewer healthy patients as having the disease (decreased false positives = increased specificity). Because the false-positive rate drops, a patient who still tests positive at this higher threshold is more likely to genuinely have the disease, resulting in an increased positive predictive value (PPV).

Step-by-Step Solution

1
Analyze the directional shift of the cutoff threshold.
Raising the cutoff threshold means a higher biomarker level is required for a test to be considered positive.
Understanding the physical shift of the threshold on a distribution curve is the foundation for determining changes in test performance.
2
Determine the impact on sensitivity and specificity.
A higher threshold leads to fewer overall positive results. This means fewer false positives (increasing specificity) but also fewer true positives and more false negatives (decreasing sensitivity).
Sensitivity and specificity always move in opposite directions when a cutoff value is shifted.
3
Determine the impact on predictive values.
Because specificity increases (fewer false positives), the proportion of true positives among all positive results increases, leading to an increased positive predictive value (PPV). Conversely, because there are more false negatives, the negative predictive value (NPV) decreases.
Predictive values are highly dependent on the rates of false positives and false negatives generated by the chosen cutoff.

Key Concept

Impact of cutoff threshold shifts on test characteristics
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Topics

Diagnostic and Screening Test Performance

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