Examkin

Question

Difficulty: MediumDiagnostic Testing Performance

A hospital infection control committee is redesigning the diagnostic algorithm for *Clostridioides difficile* infection in admitted patients who develop new-onset diarrhea. The current protocol relies entirely on a single highly sensitive glutamate dehydrogenase (GDH) antigen test. While this effectively identifies true cases, the committee notes an unacceptably high rate of false-positive diagnoses due to the detection of asymptomatic colonization. This has led to unnecessary patient isolation and inappropriate antibiotic administration. To address this issue, the committee proposes implementing a sequential testing strategy. Under the new protocol, patients will first undergo the GDH antigen test; only those with a positive GDH result will proceed to receive a highly specific toxin enzyme immunoassay (EIA). A final positive diagnosis will require both tests to be positive. Compared to using the initial highly sensitive test alone, which of the following best describes the expected impact of this new sequential testing strategy on the overall diagnostic performance metrics for the tested population?

  1. Net sensitivity decreases, and net specificity increases.Answer
  2. B
    Net sensitivity increases, and net specificity decreases.
  3. C
    Both net sensitivity and net specificity increase.
  4. D
    Both net sensitivity and net specificity decrease.
  5. E
    Net sensitivity remains unchanged, and net specificity increases.

Answer

Net sensitivity decreases, and net specificity increases.
In a sequential (series) testing strategy, a patient must test positive on all tests to be considered positive for the disease. This requirement decreases the overall (net) sensitivity because some patients with the disease will falsely test negative on the second test and be missed. However, it increases the overall (net) specificity because patients without the disease who falsely test positive on the first test have a second chance to be correctly identified as negative on the confirmatory test.

Step-by-Step Solution

1
Identify the testing strategy being implemented in the clinical scenario.
The scenario describes a sequential (series) testing strategy, where a second test is only performed if the first test is positive, and both must be positive for a final diagnosis.
Understanding the strategy is required to determine how the two tests interact mathematically.
2
Evaluate the impact of the sequential strategy on net sensitivity.
Net sensitivity decreases because true positives must now clear two testing hurdles instead of one. Any patient who falsely tests negative on the second test will be misclassified.
The net sensitivity of tests in series is the product of their individual sensitivities, which mathematically must be lower than either individual sensitivity.
3
Evaluate the impact of the sequential strategy on net specificity.
Net specificity increases because patients without the disease who falsely test positive on the first test have a second opportunity to be correctly identified as true negatives.
The second, highly specific test acts as a filter to eliminate false positives, which is the primary goal stated by the committee.

Key Concept

In sequential testing, net sensitivity decreases and net specificity increases compared to a single test. In parallel testing, net sensitivity increases and net specificity decreases.
Rate this question

Topics

Diagnostic Testing Performance

More questions from this topic