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Difficulty: MediumDiagnostic Testing Performance

An infection prevention committee at a regional transplant center is updating the screening protocol for asymptomatic carriage of carbapenem-resistant Enterobacteriaceae (CRE). The current protocol utilizes a single rapid molecular assay on perirectal swabs. While this assay is highly sensitive, its moderate specificity has led to numerous false-positive results. Consequently, many patients are unnecessarily placed in strict contact isolation, which depletes hospital resources and delays transfers.

To address this issue, the committee approves a new two-step diagnostic algorithm. Under the new protocol, patients will first undergo the rapid molecular assay. If the initial assay is positive, a second, independent phenotypic culture test will be performed. A patient will be considered a confirmed CRE carrier and placed in isolation only if both the rapid assay and the phenotypic culture yield positive results.

Compared to the previous single-test protocol, which of the following best describes the expected statistical impact of implementing this new testing algorithm on the screening pathway's overall performance characteristics?

  1. Decreased sensitivity, increased specificity, and increased positive predictive valueAnswer
  2. B
    Increased sensitivity, decreased specificity, and increased negative predictive value
  3. C
    Decreased sensitivity, increased specificity, and increased negative predictive value
  4. D
    Increased sensitivity, increased specificity, and unchanged predictive values
  5. E
    Unchanged sensitivity, increased specificity, and decreased positive predictive value

Answer

The new algorithm will result in decreased sensitivity, increased specificity, and increased positive predictive value.
The scenario describes a sequential (or series) testing strategy. In sequential testing, a patient is only considered to have the disease if both Test A and Test B are positive. This strict requirement eliminates many false positives, thereby increasing the overall specificity of the screening pathway. Because there are fewer false positives in the isolated group, the positive predictive value (PPV) also increases. However, the trade-off is an increase in false negatives (e.g., a patient with CRE who tests positive on the molecular assay but falsely negative on the culture), which consequently decreases overall sensitivity.

Step-by-Step Solution

1
Identify the type of diagnostic strategy being implemented.
The scenario describes sequential (series) testing, where a second confirmatory test is only performed if the first test is positive.
Recognizing the strategic architecture is required to determine how composite statistical metrics will shift.
2
Determine the impact of sequential testing on overall sensitivity and specificity.
Overall specificity increases because false positives from the first test are filtered out by the second test. Overall sensitivity decreases because some true positives from the first test will incorrectly test negative on the second test.
Sequential testing maximizes the ability to rule in a disease by sacrificing some ability to rule it out.
3
Deduce the downstream impact on predictive values.
Because specificity increases (yielding fewer false positives), the positive predictive value (PPV) increases. Because sensitivity decreases (yielding more false negatives), the negative predictive value (NPV) decreases.
Predictive values are mathematically tied to sensitivity, specificity, and the underlying prevalence in the tested population.

Key Concept

Sequential Testing Strategies and Performance Characteristics
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Topics

Diagnostic Testing Performance

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