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Difficulty: MediumDiagnostic Testing Performance

A hospital's quality improvement committee is reviewing the diagnostic protocol for ruling out pulmonary embolism (PE) in the emergency department. The protocol currently uses a rapid quantitative D-dimer assay with a standard positive cutoff value of 500500 ng/mL. A recent audit revealed a number of missed PE cases where patients had D-dimer levels between 250250 ng/mL and 499499 ng/mL. To prioritize identifying as many cases as possible and minimize missed diagnoses, the committee decides to lower the positive cutoff value of the D-dimer assay to 250250 ng/mL. Assuming the prevalence of PE in this patient population remains constant, which of the following pairs of test characteristics will most likely increase as a result of this protocol change?

  1. Sensitivity and negative predictive valueAnswer
  2. B
    Specificity and positive predictive value
  3. C
    Sensitivity and positive predictive value
  4. D
    Specificity and negative predictive value
  5. E
    Positive predictive value and negative predictive value

Answer

Sensitivity and negative predictive value
By lowering the D-dimer cutoff from 500500 ng/mL to 250250 ng/mL, the test becomes more sensitive. It will successfully identify more patients who actually have a pulmonary embolism (increased true positives) and miss fewer cases (decreased false negatives). Because the number of false negatives decreases, the reliability of a negative test result improves, resulting in an increased negative predictive value (NPV).

Step-by-Step Solution

1
Analyze the change in the testing threshold.
The positive cutoff is lowered from 500 ng/mL to 250 ng/mL.
Understanding the direction of the threshold shift is essential to predict the impact on diagnostic outcomes.
2
Determine the effect on true and false test results.
Lowering the cutoff means more patients will test positive. True positives (TP) and false positives (FP) will increase, while false negatives (FN) and true negatives (TN) will decrease.
Patients with values between 250 and 499 ng/mL who were previously classified as negative will now be classified as positive.
3
Apply the changes to test characteristics.
Sensitivity (TP / (TP + FN)) increases because FN decreases. Negative predictive value (TN / (TN + FN)) increases because the reduction in FN means a negative test is more likely to be a true negative. Conversely, specificity and PPV will decrease.
Sensitivity and NPV generally move in the same direction when a diagnostic threshold is adjusted to cast a wider net for disease.

Key Concept

Lowering the diagnostic cutoff threshold of a test increases its sensitivity and negative predictive value (NPV), while decreasing its specificity and positive predictive value (PPV).
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Diagnostic Testing Performance

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