In parallel testing, how is disease defined?

• A. Disease is declared if either test is positive.
• B. Disease is declared only if both tests are positive.
• C. The tests are performed sequentially.
• D. The tests are used to screen different populations.

Answer: (A)

In parallel testing, disease is defined by any positive result among the tests. Testing multiple assays at once and declaring disease if any one test is positive maximizes the chance of catching true cases, so sensitivity goes up. The trade-off is lower specificity, because a non-diseased person may test positive on at least one test, increasing false positives. By contrast, requiring all tests to be positive defines disease only if both are positive, which boosts specificity but lowers sensitivity. The remaining options describe the testing process or population strategy, not how disease status is defined in parallel testing.