Question

When a diagnostic test is used in "series" mode, then:

• A. Sensitivity increases but specificity decreases
• B. Specificity increases but sensitivity decreases
• C. Both sensitivity and specificity increase
• D. Both sensitivity and specificity decrease

Hint:

Series testing needs every test positive to call a case, so it raises specificity but lowers sensitivity; parallel testing does the opposite.

Answer 1

The Correct Option is B

Think of series testing as running two tests one after another and only calling a patient positive if both agree. Let's check each option against that rule.

1. Sensitivity increases but specificity decreases: This is the pattern for parallel testing, where any one positive test is enough to call the patient positive, not for series testing.
2. Specificity increases but sensitivity decreases: In series testing, a healthy person must fail both tests wrongly to be labelled positive, which is rare, so false positives drop and specificity climbs. A truly sick person only needs one false negative anywhere in the sequence to be missed, so sensitivity falls.
3. Both sensitivity and specificity increase: Combining tests always trades one property for the other in a fixed protocol, it cannot raise both at once.
4. Both sensitivity and specificity decrease: This does not match either series or parallel logic; at least one measure always improves with combined testing.

So series testing pushes specificity up while sensitivity comes down, since a case is confirmed only when every test in the chain is positive.

Let's summarize:

• Series testing: needs all tests positive, raises specificity, lowers sensitivity, good for confirming a diagnosis.
• Parallel testing: needs any one test positive, raises sensitivity, lowers specificity, good for not missing a case.

The correct answer is that specificity increases while sensitivity decreases.