If Hemoccult test is negative for screening of colonic cancer, no fuher test is done. If the hemoccult test is positive the individual will have a second stool sample tested with Hemoccult II test. If this second sample is positive for blood, the individual will be referred for more extensive evaluation. The effect of net sensitivity and net specificity of this method of screening is

## Core Concept
The question assesses understanding of the impact of sequential testing on the sensitivity and specificity of a screening method for colonic cancer. The Hemoccult test is used as a primary screening tool, followed by the Hemoccult II test for those who test positive. This approach involves understanding how two-step testing affects the overall sensitivity and specificity of the screening process.

## Why the Correct Answer is Right
The correct answer, , indicates that the sequential testing strategy decreases the sensitivity but increases the specificity.
- **Sensitivity** refers to the test's ability to correctly identify those with the disease (true positive rate).
- **Specificity** refers to the test's ability to correctly identify those without the disease (true negative rate).
In a two-step testing strategy like this, where a positive result on the first test is followed by a second test:
- A decrease in **sensitivity** occurs because some individuals with the disease might test negative on the second test, even if they tested positive on the first.
- An increase in **specificity** occurs because those without the disease are less likely to test positive on both tests, reducing false positives.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests an increase in sensitivity and specificity, which is not accurate for a sequential testing strategy that requires two positive tests to refer for evaluation. The sensitivity decreases because some true positives may be missed in the second test.
- **Option B:** This option suggests an increase in sensitivity and a decrease in specificity, which is the opposite of what occurs. The specificity increases because the strategy becomes more selective.
- **Option D:** This option suggests a decrease in both sensitivity and specificity, which is incorrect because the specificity increases with sequential testing.

## Clinical Pearl / High-Yield Fact
A key point to remember is that sequential testing, or using a second, more specific test after a positive initial screen, **increases the positive predictive value (PPV) of the testing strategy**. This means that a positive result from the two-step process is more likely to be a true positive. However, it's crucial to balance this with the potential decrease in sensitivity, ensuring that not too many cases are missed.

## Correct Answer: C.

✓ Correct Answer: B. Net sensitivity is decreased and net specificity is increased