Lecture 16

Question 1

Even though it is necessary, the necessary cause may not

Answer 1

result in the disease if the other stuff is there

Question 2

Define screening (in health)

Answer 2

involves identifying risk factors for disease or unrecognised disease by applying tests on a large scale to a population.

Question 3

Is screening primary, secondary or tertiary prevention?

Answer 3

It can be either one of the three depending on what they are screening for

Question 4

When is screening a primary prevention

Answer 4

Before the biological onset, when they are screening for a risk factor (alcohol in women)

Question 5

When is screening a secondary prevention

Answer 5

After the biological onset, when they are looking early features of disease before the disease gets worse (mammograms)

Question 6

When is screening a tertiary prevention

Answer 6

Following the diagnosis and treatment of the disease, when they are screening for a complication of a diagnosed disease (bone density)

Question 7

What is the flow diagram for a screening program?

Answer 7

First they take the total population and find an eligible population without symptoms. Then subject them to a screening test that is less expensive and can identify probable cases of the disease. Test positives are then subjected to invasive and expensive gold standard test. Then the disease positive from this group receive intervention/treatment. The disease negative will be rescreened after a specified amount of time.

Question 8

How do they find the eligible population group

Answer 8

through descriptive epidemiological studies

Question 9

What groups does the screening test identify

Answer 9

test positive and test negative

Question 10

Define gold standard test

Answer 10

The best diagnostic test they have to offer- the ideal test.

Question 11

Why can’t we subject large populations to gold standard tests?

Answer 11

They are often invasive and expensive.

Question 12

For a successful screening initiative (criteria)

Answer 12

suitable disease, test, treatment

Question 13

The objective of screening is to

Answer 13

To improve health outcome for a population

Question 14

What is a suitable disease

Answer 14

It has to be an important health problem. The natural history of the disease has to be known/ or relationship of risk factors known.

Question 15

Can diseases chosen be common or uncommon ?

Answer 15

Both. For uncommon diseases that are easily detectable and easily intervened, they are screened as well

Question 16

For natural history of disease what about the timing of the disease?

Answer 16

It needs to be detectable early. The pre-clinical phase (early features to diagnosis) has to be long enough for the screening program to identify early feature and treat it before it gets worse

Question 17

What are features of a Suitable Test

Answer 17

It has to be an easy test that can be applied to a large population. It has to be reliable- giving consistent results. It has to safe, simple, effective and clearly accurate.

Question 18

To be an effective test it has to be

Answer 18

affordable and acceptable to the population you’re delivering it so they all participate in the screening programme

Question 19

What is measures of accuracy

Answer 19

Specificity and sensitivity

Question 20

When you want to test the accuracy of a screening test, you compare it against a

Answer 20

gold standard test

Question 21

What is sensitivity

Answer 21

The likelihood of a positive test in those who actually have the disease. How many people did the test identify had the disease out of the people who actually have the disease?

Question 22

How do you calculate sensitivity

Answer 22

(true positives/ all those who have the disease (EG) ) x 100

Question 23

What is specificity

Answer 23

The likelihood of a negative test in those who actually did Not have the disease. How many people did the test say did not have the disease that actually didn’t have the disease?

Question 24

How do you calculate specificity

Answer 24

(true negatives/ all those who don’t have the disease (CG) ) x 100

Question 25

What units are specificity / sensitivity given in

Answer 25

percentage

Question 26

Sensitivity of a screening test is high if

Answer 26

the proportion of true positives is high

Question 27

Specificity of a screening test is high if

Answer 27

the proportion of true negatives is high

Question 28

Wherever we apply the test, the results of specificity and sensitivity are a

Answer 28

fixed value characteristic of the test.

Question 29

If you have the same population size, sensitivity and specificity values, why can they pick up different positive tests

Answer 29

They is a difference in the prevalence of the disease in the populations (EG and CG)

Question 30

What are predictive values

Answer 30

Another measure of accuracy of the screening test in practice. This looks at the number of people that are testing positive and testing negative

Question 31

Positive predictive value (PPV) is

Answer 31

the probability of having the disease if the test is positive

Question 32

What is the main difference between predictive values and specificity/sensitivity

Answer 32

predictive values look across the outcome grid whereas specificity/sensitivity go vertically up.

Question 33

How do you calculate PPV

Answer 33

((true positives/ all those who test positive)) x 100

Question 34

Negative predictive value (NPV) is

Answer 34

The probability of being free from the disease if the test is negative

Question 35

How do you calculate NPV

Answer 35

((true negatives/ all those who test negative)) x 100

Question 36

Are predictive values fixed values of the test?

Answer 36

No, they are dependent on the prevalence of disease in the population. So they reflect both test accuracy and the prevalence of the disease in the population

Question 37

What are the requirements of suitable treatment?

Answer 37

You have to have evidence that early treatment leads to better health outcomes. The treatment has to be effective, acceptable and accessible. There has to be evidence based policies that regulate who is offered treatment and what treatment is offered so as not to cause disparities in healthcare utilisation- therefore health outcomes for population groups

Question 38

What are the requirements of a suitable screening programme

Answer 38

The benefits must outweigh the harm. There must be RCT evidence that the screening programme reduces mortality or increases survival time.
There must be adequate resourcing, agreed policy for testing, diagnosis, treatment and programme management with health care support throughout. Cost effective. Needs to reach all those who are likely to benefit from it.

Question 39

What are the two biases involved in calculating survival time

Answer 39

lead time bias and length time bias

Question 40

Define lead bias

Answer 40

This happens when the time of survival appears to be longer even though the time of death is the same for both when there was screening and when there was not, because the time it was measured from is earlier (screening) than it would’ve been without screening (diagnosis)

Question 41

Define length time bias

Answer 41

If there is variation in the speed of progression of the disease, a mean survival time can be taken but this may lead to a bias towards longer survival times because the people with faster progressing diseases may have already died (so not counted) or have yet to start.