## Core Concept
When screening tests are used in series, both tests must be positive for a person to be considered positive for the disease. This approach increases the specificity of the testing but decreases the sensitivity.

## Why the Correct Answer is Right
In a series testing strategy, the overall sensitivity is the product of the sensitivities of the individual tests (i.e., sensitivity 1 × sensitivity 2), and the overall specificity is 1 minus the product of (1 - specificity 1) × (1 - specificity 2). This results in an increase in specificity but a decrease in sensitivity. The correct formula reflecting an increase in specificity when tests are used in series is therefore related to option .

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a decrease in both sensitivity and specificity, which is not accurate for specificity when tests are used in series.
- **Option B:** This option implies no change, which does not reflect the actual outcome of using tests in series.
- **Option D:** This option suggests an increase in sensitivity, which is the opposite of what occurs when tests are used in series.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that using tests in series is a strategy to confirm a diagnosis, particularly when the disease is severe and the cost of a false positive is high. It is essential to understand that while series testing increases the confidence in a positive diagnosis (increased specificity), it may miss some actual cases (decreased sensitivity).

## Correct Answer: C.