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Difficulty: HardDiagnostic and Screening Test Performance

A clinical research committee is evaluating a newly developed quantitative immunoassay that detects a circulating tumor biomarker for early pancreatic adenocarcinoma. The current diagnostic threshold for a positive test is set at 150 ng/mL. In an effort to minimize the number of missed early-stage malignancies, the committee decides to adjust the threshold for a positive result to 75 ng/mL. When implemented in the same screening population, which of the following pairs of statistical parameters will most likely both decrease as a direct result of this threshold adjustment?

  1. A
    Sensitivity and positive predictive value
  2. B
    Specificity and negative predictive value
  3. False-negative rate and positive predictive valueAnswer
  4. D
    False-positive rate and negative predictive value
  5. E
    True-negative rate and false-positive rate

Answer

The false-negative rate and positive predictive value will both decrease.
Lowering the diagnostic threshold (from 150 ng/mL to 75 ng/mL) means a lower concentration of the biomarker is required for a positive result. This change makes the test more 'sensitive,' correctly identifying more patients who actually have the disease (increased true positives) and missing fewer patients with the disease (decreased false negatives). Consequently, the false-negative rate decreases. However, it will also incorrectly label more healthy individuals as having the disease (increased false positives). Because the number of false positives increases, the proportion of positive test results that are truly positive drops, leading to a decreased positive predictive value (PPV). Therefore, both the false-negative rate and the positive predictive value decrease.

Step-by-Step Solution

1
Identify the direction of the threshold shift.
The threshold is lowered from 150 ng/mL to 75 ng/mL.
To determine how the absolute number of positive and negative test results will change.
2
Determine the effect on true and false results.
Lowering the threshold makes it easier for a test to be positive. This increases both true positives (TP) and false positives (FP), while decreasing true negatives (TN) and false negatives (FN).
Understanding these raw changes is required to calculate derived test characteristics.
3
Analyze the impact on the parameters in the correct option.
Because false negatives (FN) decrease, the false-negative rate decreases. Because false positives (FP) increase, the proportion of positive results that are actually true positives drops, leading to a decreased positive predictive value (PPV).
To identify the specific pair of parameters that both decrease in this scenario.

Key Concept

Effect of shifting diagnostic thresholds on test characteristics
Estimated Time:1m 30s
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Topics

Diagnostic and Screening Test Performance

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