Which statement correctly describes the effect of a series interpretation on sensitivity and specificity?

• A. Increases sensitivity but decreases specificity
• B. Increases specificity but decreases sensitivity
• C. Increases both sensitivity and specificity
• D. Decreases both sensitivity and specificity

Answer: (B)

Series interpretation requires both tests to be positive before labeling someone as diseased. This tightens the positive criterion, so fewer healthy people are incorrectly labeled positive, which increases specificity. But because a diseased person could be positive on one test and negative on the other, some true cases are missed, lowering sensitivity. If the two tests are treated as independent, overall sensitivity is roughly the product of their sensitivities, while overall specificity is higher than either test alone. For example, two tests with about 90% sensitivity and 95% specificity together tend to give around 80%–81% sensitivity but around 99% specificity. So the effect of series interpretation is to increase specificity while decreasing sensitivity.