1. Chapter 2, Unit E
Screening Tests

2. Essential Question:
How accurate are the tests claiming to be REALLY?

3. Screening Tests: Used to test a large population for a disease
Must be cheap and easy to apply to a large number of people
Not usually very precise
If a screening test comes back positive, the person is tested again, using a more accurate confirmatory test

4. Screening Test Questions:
Does the person actually have the disease?
Will the person test positive or negative?

5. Screen Test Results= Could be in 4 Groups:
False negative: Have the disease but test negative
True positive: Have the disease and test positive
True negative: Don’t have the disease and test negative
False positive: Don’t have the disease but test positive

6. Example 1: HIV Screening
United States
Tests Positive
Tests Negative
Total
Has HIV
5,982 18
6,000
Doesn’t have HIV
14,910
979,090
994,000
20,982
979,108
1,000,000

7. Example 1: HIV Screening
What percentage of people in the US have HIV? This is the PREVELANCE of the disease.
What percentage of the people with HIV actually test positive? (round your percentage to one decimal place)
What percentage of people without HIV test positive? This is called the FALSE POSITIVE rate.
What percentage of people with HIV test negative? This is called the FALSE NEGATIVE rate.
What percentage of people who test positive actually have HIV? This is the PREDICTIVE VALUE OF A POSITIVE TEST and answers the question, “If someone tests positive, what is the probability that they actually have HIV?”

8. Example 1: HIV Screening
What percentage of people in the US have HIV? This is the PREVELANCE of the disease. 0.6%
What percentage of the people with HIV actually test positive? (round your percentage to one decimal place) 99.7%
What percentage of people without HIV test positive? This is called the FALSE POSITIVE rate. 1.5%
What percentage of people with HIV test negative? This is called the FALSE NEGATIVE rate. 0.3%
What percentage of people who test positive actually have HIV? This is the PREDICTIVE VALUE OF A POSITIVE TEST and answers the question, “If someone tests positive, what is the probability that they actually have HIV?” 28.5%

9. Example 2: HIV Screening Botswana
Tests Positive
Tests Negative
Total
Has HIV
238,283
717
239,000
Doesn’t have HIV
11,415
749,585
761,000
249,698
750,302
1,000,000

10. What percentage of people in Botswana have HIV?
What percentage of people without HIV test positive? FALSE POSITIVE RATE
What percentage of people with HIV test negative? FALSE NEGATIVE RATE
What percentage of people who test positive actually have HIV? PREDICTIVE VALUE OF POSITIVE TEST
Compared to the data from the US, which of the computed values changed and which stayed the same?

11. What percentage of people in Botswana have HIV? 23.9%
What percentage of people without HIV test positive? FALSE POSITIVE RATE 4.6%
What percentage of people with HIV test negative? FALSE NEGATIVE RATE 0.1%
What percentage of people who test positive actually have HIV? PREDICTIVE VALUE OF POSITIVE TEST 99.7%
Compared to the data from the US, which of the computed values changed and which stayed the same?