EIP - Screening and Diagnostic Tests - Week 11

Question 1

Define screening test.

Answer 1

Screening is a public health service in which members of a defined population, who do not necessarily perceive they are at risk of, or are already affected by a disease or its complications, are asked a question or offered a test, to identify those individuals who are more likely to be helped than harmed by further tests or treatment to reduce the risk of a disease or its complications.

Question 2

Only when what criteria is met should screening be introduced?

Answer 2

Screening should only be introduced when the balance of benefits vs. harms and costs is favourable, and this has been demonstrated by sound evaluation

Question 3

Define a 100% sensitive test.

Answer 3

A 100% sensitive test is always positive in subjects in whom the disease of interest is present
-if you have the disease and you do this test, it will always pick it up (every case is picked up)

Question 4

Define a 100% specificity test.

Answer 4

A 100% specific test is only positive when the disease of interest is present
Always negative when the disease is absent

Question 5

Define sensitivity.

Answer 5

Those who test positive out of those with the disease

Question 6

Define specificity.

Answer 6

Those who test negative out of those without the disease

Question 7

Define positive predictive value. When do you want to know this?

Answer 7

The probability of having the disease when the test is positive
This is what you want to know when you screen someone

Question 8

Define negative predictive value.

Answer 8

The probability of not having the disease when the test is negative

Question 9

How does higher prevalence affect positive and negative predictive values?

Answer 9

It increases positive predictive values and decreases negative predictive values

Question 10

What 5 things would pre-test probability estimates depend on?

Answer 10

Own clinical experience
Regional/national prevalence statistics
Databases
Accuracy and importance of the test
Specific studies that determine pre-test probabilities

Question 11

What may happen if the pre-test probability is high but you dont get a positive on the test?

Answer 11

Might move on and confirm with a more accurate test

Question 12

What does a likelihood ratio for a positive test result show?

Answer 12

How much more likely a patient’s positive test result would be for someone with the disease compared to someone without the disease

Question 13

What does a likelihood ratio for a negative test result show?

Answer 13

How much more likely a patient’s negative test result would be for someone without the disease compared to someone with the disease

Question 14

Briefly explain how to handle continuous data in the context of defining a disease? Give an example with IOP and glaucoma.

Answer 14

Must dichotomise IOP results by setting a criterion below which IOP is normal, and above which indicates glaucoma.
Can be set to where one thinks it will be most useful.
Collect a large set of IOPs and whether or not they have glaucoma.
Pick an arbitrary level for IOP, and draw a 2x2 table
-how many had <21mmHg with/without glaucoma
-how many had >21mmHg with/without glaucoma

Question 15

Explain the receiver operating characteristic curve. What is at the y- ans x-axes?
What is the best criterion to use?

Answer 15

It plots sensitivity as a function of 1-specificity for different criteria.
y-axis is sensitivity
x-axis is 1 - specificity
The best criteria to use is one that gives the best combination of specificity and sensitivity - in other words, the greater the area under the graph, the better (the closer it is to the top-left corner of the graph)