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Difficulty: EasyDiagnostic Testing Performance

An internal medicine physician is reviewing the utility of D-dimer testing for evaluating suspected pulmonary embolism (PE). The physician considers two different clinical scenarios. Patient X is a healthy 25-year-old with atypical chest pain and a modified Wells score of 0, yielding a very low pretest probability for PE. Patient Y is a 65-year-old with active cancer, recent orthopedic surgery, and hemoptysis, yielding a high pretest probability for PE. If the identical D-dimer assay is applied to both patients, which of the following diagnostic parameters will be mathematically higher for Patient X compared to Patient Y?

  1. Negative predictive valueAnswer
  2. B
    Positive predictive value
  3. C
    Sensitivity
  4. D
    Specificity
  5. E
    Positive likelihood ratio

Answer

Negative predictive value
The correct answer is Negative predictive value. In clinical practice, pretest probability functions identically to disease prevalence. A fundamental rule of biostatistics is that as the pretest probability (prevalence) of a disease decreases, the Negative Predictive Value (NPV) of a test increases, and the Positive Predictive Value (PPV) decreases. Because Patient X has a very low pretest probability compared to Patient Y, the NPV of the D-dimer test will be mathematically higher for Patient X. The other parameters listed are intrinsic to the test and do not change.

Step-by-Step Solution

1
Determine the difference in baseline disease likelihood between the two scenarios.
Patient X has a low pretest probability (low prevalence setting), while Patient Y has a high pretest probability (high prevalence setting).
Understanding the baseline risk is required to determine how predictive values will shift.
2
Recall the relationship between pretest probability (prevalence) and diagnostic test characteristics.
Intrinsic parameters (sensitivity, specificity, likelihood ratios) do not change. Extrinsic parameters (predictive values) change with prevalence.
This establishes which options can be immediately eliminated.
3
Apply the rule for predictive values to Patient X's low pretest probability.
As pretest probability decreases, Positive Predictive Value (PPV) decreases and Negative Predictive Value (NPV) increases.
This identifies the specific parameter that will be higher in the low-risk patient.

Key Concept

The effect of pretest probability (prevalence) on Negative Predictive Value
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Topics

Diagnostic Testing Performance

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