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Difficulty: MediumDiagnostic and Screening Test Performance

An emergency medicine physician is evaluating a 68-year-old man who presents with pleuritic chest pain and shortness of breath. The physician decides to order a D-dimer assay to help rule out a pulmonary embolism. The laboratory recently updated its reporting system: instead of using a uniform threshold of 500 ng/mL500 \text{ ng/mL} for all patients, it now applies an age-adjusted threshold (calculated as age×10 ng/mL\text{age} \times 10 \text{ ng/mL}) for individuals over 50 years of age. For this patient, a result is now considered positive only if it exceeds 680 ng/mL680 \text{ ng/mL}. Compared to the previous uniform standard, how does implementing this higher threshold affect the diagnostic performance of the D-dimer test in the demographic of patients older than 50?

  1. Decreased sensitivity and increased positive predictive valueAnswer
  2. B
    Decreased sensitivity and decreased positive predictive value
  3. C
    Increased sensitivity and decreased positive predictive value
  4. D
    Increased sensitivity and increased positive predictive value
  5. E
    No change in sensitivity and increased positive predictive value

Answer

The implementation of a higher diagnostic threshold will result in decreased sensitivity and increased positive predictive value.
Raising the cutoff value for a positive test means that a higher D-dimer concentration is required to diagnose the condition. This inherently causes more patients with the disease to test negative (increased false negatives), which directly decreases the test's sensitivity. Simultaneously, it causes fewer healthy patients to falsely test positive (decreased false positives), which increases the test's specificity. Because a positive result now requires a much more extreme value, the likelihood that a positive result truly indicates the presence of the disease goes up. Therefore, despite the drop in sensitivity, the massive reduction in false positives leads to an overall increase in the positive predictive value (PPV).

Step-by-Step Solution

1
Analyze the effect of raising the cutoff threshold on true positives and false negatives.
Fewer patients with pulmonary embolism will test positive, leading to decreased true positives and increased false negatives.
A higher threshold requires a more extreme biomarker value to be considered positive, inevitably missing milder cases of the disease.
2
Determine the impact on sensitivity based on the changes in true positives and false negatives.
Sensitivity decreases.
Sensitivity is the proportion of diseased individuals correctly identified. Since true positives decrease and false negatives increase, the overall sensitivity decreases.
3
Analyze the effect of raising the cutoff on false positives and positive predictive value (PPV).
False positives decrease significantly, which leads to an increase in PPV.
PPV is the proportion of positive results that are true positives. Although true positives decrease slightly, the number of false positives drops drastically, resulting in a net mathematical increase in the PPV.

Key Concept

Effect of shifting diagnostic cutoffs on test performance characteristics
Estimated Time:1m 15s
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Topics

Diagnostic and Screening Test Performance

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