lecture 35 - screening Flashcards
what is screening
the widespread use of a simple test for a disease in an apparently healthy population
what is a screening program
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
what is a screening test
- 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
why try detect a disease early
secondary prevention method
what is a screening test NOT
not a diagnostic test
not a screening program
key concepts of screening
aims to improve outcomes, usually to reduce mortality
all screening programs do harm, some can do good as well
what is a case finding (or opportunistic screening) and how is this different to a screening program
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
what are the factors determining if the disease is appropriate for screening
- seriousness of the disease
- prevalence of pre clinical disease
- lead time
- ability to alter course of the disease
why does the serious of disease matter when determining if screening is appropriate
screening is resource intensive so makes sense to screen for diseases with potentially severe consequences
- consider mortality, morbidity and disability
why does the prevalence of pre clinical disease matter when determining if screening is appropriate
more efficient when there is a high prevalence of the preclinical stage of the disease
why does the lead time of disease matter when determining if screening is appropriate
length of lead time determines the screening interval
long lead time = greater chance of detecting disease early
why does the change course of disease matter when determining if screening is appropriate
- has to be some effective therapy of treatment available
- screening has to improve the length and / or quality of someones life
what determines if the screening test is appropriate
- is the test accurate
- is the test acceptable and safe
how can you determine the accuracy of a screening test
sensitivity and specificity
what is sensitivity with determining the accuracy of a screening test
proportion of people with the disease who test positive
- tells us how good a test is at identifying those with the disease
how is sensitivity calculated with determining the accuracy of a screening test
true positives / diseases status (all those with the disease)
what is specificity with determining the accuracy of a screening test
proportion of people without the disease who test negative
- tells us how good the test is at identifying those without the disease
how is specificity calculated with determining the accuracy of a screening test
true negatives / all of those without the disease
we wan to maximise both specificity and sensitivity how is this done
- improving the screening test
- choice of disease threshold
BUT ….
- there is a trade of between sensitivity and specificity
what are predictive values
measure test performance in a particular population
- what proportion of people who test positive / negative do / dont have the disease
what is a positive predictive value and how is it calculated
proportion of people who test positive and have the disease
true positives / all of the people that have positive test
what is a negative predicted value and how is it calculated
proportion of people who test negative and dont have the disease
true negatives / all of the people who test negative
unlike sensitivity and specificity, what are positive and negative predicted values influences by
influenced by disease prevalence in the population
what are the factors determining if a screening program would be effective
- are there resources to implement and cope with positives
- is the programme actually effective
what are the benefits for screening programs that must outweigh the harms
- potential for early detection and intervention = reduced mortality / morbidity
- reassurance (true negatives)
- improved health of the population
what are the harms of screening programs
- increases in health iniquities from unequal participation or treatment
- physical - from complications, invasive tests and/or treatments
- physiological - from anxiety from waiting, distress from invasive procedures, knowing about serious diagnosis for longer
- financial - to individual or health service
