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

Step 1: Understanding the Question.
The question asks what happens to sensitivity and specificity when two diagnostic tests are combined in series, meaning both must be positive before the patient is called a case.

Step 2: Key Concept or Approach.
In series testing, a person is called positive only if every test in the sequence comes back positive. A single negative result anywhere in the sequence rules the patient out. This is different from parallel testing, where a person is called positive if any one test is positive, and a case is ruled out only if all tests are negative.

Step 3: Detailed Explanation.
Because series testing needs every test to agree before calling someone positive, it becomes harder for a healthy person to be wrongly labelled positive on all tests together, so false positives fall and specificity goes up. At the same time, a truly diseased person only needs one test in the sequence to come back falsely negative, and they are then classed as negative overall, so more true cases get missed and sensitivity falls. Series testing is therefore used to confirm a diagnosis, cut down unnecessary treatment, and save cost, typically ordering the cheaper or simpler test first and the more specific or expensive test second. Parallel testing works the opposite way. Because a person only needs one positive test out of several, sensitivity rises (few true cases are missed) but specificity falls (more healthy people end up with at least one false positive), which is why parallel testing suits emergency situations where missing a case is dangerous.

Step 4: Final Answer.
Series testing raises specificity and lowers sensitivity, so the correct statement is that specificity increases while sensitivity decreases.