lecture 35 - screening

Question 1

what is screening

Answer 1

the widespread use of a simple test for a disease in an apparently healthy population

Question 2

what is a screening program

Answer 2

an organised system using a screening test among asymptomatic people in the population to identify early cases of the disease in order to improve outcomes

Question 3

what is a screening test

Answer 3

• usually relatively cheap and simple
• used to test a large numbers of apparently healthy people to identify people who are expected to have early disease
• these people will go on to have further diagnostic tests to confirm the diagnosis

Question 4

why try detect a disease early

Answer 4

secondary prevention method

Question 5

what is a screening test NOT

Answer 5

not a diagnostic test

not a screening program

Question 6

key concepts of screening

Answer 6

aims to improve outcomes, usually to reduce mortality

all screening programs do harm, some can do good as well

Question 7

what is a case finding (or opportunistic screening) and how is this different to a screening program

Answer 7

case finding = when you go to the doctor for swollen ankle and they check your blood pressure while you are there and find that it is abnormal

Question 8

what are the factors determining if the disease is appropriate for screening

Answer 8

• seriousness of the disease
• prevalence of pre clinical disease
• lead time
• ability to alter course of the disease

Question 9

why does the serious of disease matter when determining if screening is appropriate

Answer 9

screening is resource intensive so makes sense to screen for diseases with potentially severe consequences

• consider mortality, morbidity and disability

Question 10

why does the prevalence of pre clinical disease matter when determining if screening is appropriate

Answer 10

more efficient when there is a high prevalence of the preclinical stage of the disease

Question 11

why does the lead time of disease matter when determining if screening is appropriate

Answer 11

length of lead time determines the screening interval

long lead time = greater chance of detecting disease early

Question 12

why does the change course of disease matter when determining if screening is appropriate

Answer 12

• has to be some effective therapy of treatment available
• screening has to improve the length and / or quality of someones life

Question 13

what determines if the screening test is appropriate

Answer 13

• is the test accurate
• is the test acceptable and safe

Question 14

how can you determine the accuracy of a screening test

Answer 14

sensitivity and specificity

Question 15

what is sensitivity with determining the accuracy of a screening test

Answer 15

proportion of people with the disease who test positive
- tells us how good a test is at identifying those with the disease

Question 16

how is sensitivity calculated with determining the accuracy of a screening test

Answer 16

true positives / diseases status (all those with the disease)

Question 17

what is specificity with determining the accuracy of a screening test

Answer 17

proportion of people without the disease who test negative
- tells us how good the test is at identifying those without the disease

Question 18

how is specificity calculated with determining the accuracy of a screening test

Answer 18

true negatives / all of those without the disease

Question 19

we wan to maximise both specificity and sensitivity how is this done

Answer 19

• improving the screening test
• choice of disease threshold

BUT ….

• there is a trade of between sensitivity and specificity

Question 20

what are predictive values

Answer 20

measure test performance in a particular population

• what proportion of people who test positive / negative do / dont have the disease

Question 21

what is a positive predictive value and how is it calculated

Answer 21

proportion of people who test positive and have the disease

true positives / all of the people that have positive test

Question 22

what is a negative predicted value and how is it calculated

Answer 22

proportion of people who test negative and dont have the disease

true negatives / all of the people who test negative

Question 23

unlike sensitivity and specificity, what are positive and negative predicted values influences by

Answer 23

influenced by disease prevalence in the population

Question 24

what are the factors determining if a screening program would be effective

Answer 24

• are there resources to implement and cope with positives
• is the programme actually effective

Question 25

what are the benefits for screening programs that must outweigh the harms

Answer 25

• potential for early detection and intervention = reduced mortality / morbidity
• reassurance (true negatives)
• improved health of the population

Question 26

what are the harms of screening programs

Answer 26

1. increases in health iniquities from unequal participation or treatment
2. physical - from complications, invasive tests and/or treatments
3. physiological - from anxiety from waiting, distress from invasive procedures, knowing about serious diagnosis for longer
4. financial - to individual or health service